More than half of all TV viewers tuned in to Girls Aloud star Cheryl's debut as a judge - 1.5million more than in 2007.
A fellow new girl sexy Holly Willoughby was also a hit - netting show spin-off Xtra Factor a best ever 1.5MILLION audience.
Simon Cowell said: "We're back bigger and better than before. It really is the best series we've done."
The figures are a boost for ITV after a run of poor ratings.
An insider said: "It shows there is still a huge appetite for the X Factor, even though it's the fifth series."