Polling closes at 5pm, bringing to an end a long, fractious contest that has dominated the summer and saw both candidates clash over competing visions for the country.
Foreign Secretary Ms Truss is believed to be on course for victory and in her final message on Friday she paid tribute to the party members she had met in recent months.
She said: “It has been fantastic meeting and speaking to thousands of members across the whole of the UK over the last six weeks.
“Our members make our party great and I would like to thank all the volunteers who have helped along the way.
“I believe in a brighter and better future for Britain. I have a bold plan that will grow our economy and deliver higher wages, more security for families and world-class public services.
“I’ll do this by cutting taxes, pushing through supply-side reform and slashing red tape that is holding businesses back.
“If I am elected prime minister, I will never let anyone talk us down and I will do everything in my power to make sure our great nation succeeds.”
Mr Sunak said he was “humbled” by his engagement with the party faithful.
In a statement, he said: “I have been humbled to meet so many thousands of our brilliant members and activists over the past six weeks.
“This is a critical election for our country and for the future of the Conservative Party, as we eye a historic fifth term in government.
“We face huge challenges ahead, but also huge opportunities. I know what it takes to get through challenging times. I did it as chancellor and I will do it again as prime minister.”
After voting closes, Boris Johnson’s successor will be announced on Monday, taking over as prime minister the following day.
Earlier Lord Hayward, a former MP and current Tory peer, said he is not convinced Ms Truss’s victory will be by such an emphatic margin, but she remains on course to become the next prime minister.
He said a tighter win will mean it is “absolutely necessary” for Ms Truss to appoint a cabinet that brings together all sides of the Tory parliamentary party.
Lord Hayward told the PA news agency: “My overall sense is Liz Truss will win but I am not convinced it will be by the margin that the polls are predicting.”
He said his prediction of a tighter margin of victory for Ms Truss is rooted in the conversations he has had with those who have attended the hustings.
On how the campaign has played out, Lord Hayward said: “The party and the individuals will be bruised by the experience, there’s no question, and it will take time to recover.
“At the moment it appears from all the leaks that all the positions seem to be going to Truss supporters in the cabinet.
“Now that would be bad even if there was a very clear margin of victory, but if there’s a narrower margin of victory it is absolutely necessary that right at the top the sides are brought together.
“Liz Truss starts off with a disadvantage which has faced a number of other politicians in their time – the most recent one being probably Jeremy Corbyn – in that she doesn’t necessarily have the automatic support of the majority of her parliamentary party.”
Mr Sunak has consistently acknowledged he is the underdog and his supporters continue to hope he can cause a surprise.
Kevin Hollinrake, Conservative MP for Thirsk and Malton, told Sky News: “I’ve seen some of the polls and national polls. I think it’s quite hard for pollsters to determine who is a Conservative member and who is not because there’s not an open database.
If there's a narrower margin of victory it is absolutely necessary that right at the top the sides are brought together
“But I know who mine are. I polled my 700 members, 239 of them responded, so about a third of them responded, and Rishi got an eight-point lead.
“I’ve seen similar kind of polls around different constituencies around the country. So I don’t think he’s cut and dried. I think he’s probably neck and neck.”
The leadership contest has been characterised by infighting among Conservative MPs, with blue-on-blue attacks continuing up until the final days.
Mr Johnson has sought to use his final weeks to outline what he perceives to be his legacy from his time in Downing Street.
His term of office was rocked by the partygate scandal and was ultimately brought to an end over the way he handled allegations of inappropriate behaviour by former Conservative whip Chris Pincher.
Fresh allegations of misconduct were brought to light on Thursday, with Sky News reporting that one woman had been assaulted by a Cabinet minister, while another was groped by a senior No 10 aide.
Mr Johnson and his successor will then go to Balmoral rather than Buckingham Palace for the appointment of the new prime minister, in a break from tradition.
The Queen will receive Mr Johnson on Tuesday September 6 at her Aberdeenshire home, where he will formally tender his resignation.
This will be followed by an audience with the new Tory leader, where she or he will be invited to form a government.