Sir Winston Churchill finished second, with JK Rowling third.
But just a couple of hundred votes separated Diana from JK!
They were well ahead of footballers David Beckham and Michael Owen.
In the adult vote, Churchill came first with Princess Diana back in third.
Why did you vote for Diana?
Jessica from Buckinghamshire says "she was nice and kind to children in hospital", while Angie from Lincolnshire says "she was caring and sensitive".
There were two kinds of votes, one on the CBBC Newsround website and a telephone one plugged on the TV programme.
On the website JK Rowling was an easy winner, but Princess Diana got loads more votes on the phone lines, edging her clear of the Harry Potter author.