The Oricon Communications market survey firm reports that Eiichiro Oda's entire One Piece pirate manga series has sold over 20 million copies — just in 2010. As of the August 16-22 week, the manga series has sold 20.328 million copies. This is the first time that any series in any book category has sold that many copies in one year since Oricon began posting its Japanese book ranking charts in 2008. The second highest selling manga series is Naruto, which has sold over 15 million copies.

The last three volumes of One Piece, #57 through #59, have each sold over 2 million copies. In particular, volumes 57 (1.691 million copies) and 59 (1.853 million copies) each have held the record for the most books sold in the first week.

source: Oricon

Update: One Piece outsold Naruto by over 15 millon copies; Naruto did not sell 15 million copies.