Armstrong will break his silence over damning accusations of his drug doping on the Oprah show a week today.
But Millar, a member of the athletes' commission for the World Anti-Doping Agency, said the American needs to come clean in a proper arena.
A spokeswoman for the Oprah show said Armstrong was not being paid to appear and that Winfrey was free to ask him any question she wanted.
But Millar said: "Only Lance would get to have his moment of truth, if that's what it will be, in front of Oprah Winfrey.
"It is not sitting in front of a judge or a disciplinary hearing being properly questioned about the things he has done wrong. I doubt very much it will be a proper interrogation."
Millar himself served a two-year ban after admitting doping in 2004, since when he has become a campaigner against drugs in sport.
ICE HOCKEY: Braehead Clan coach Jordan Krestanovich has left the door open for Drew Miller to return if plans for an NHL season fall through.
He said: "As soon as he came in, he just slotted in like one of the guys. It was great to have a guy like him on the team.
"If the NHL season doesn't go ahead for any reason, then he knows where we are."