Blatter said that the technical report into Qatar's bid had warned that it was too hot but despite that, FIFA's executive committee had voted for the Middle Eastern country.
The FIFA president said it was now "probable" that it would be played in the winter rather than the summer due to the heat.
Blatter told Swiss TV station RTS: "Yes, it was a mistake of course, but one makes lots of mistakes in life.
"The technical report into Qatar said clearly it was too hot but the executive committee - with a large majority - decided all the same to play it in Qatar."
Blatter insisted however that Qatar had not "bought" the World Cup - the bid did spend large amounts of money on sponsorship and development programmes.
He added: "No, I have never said it was bought, but that it was due to political considerations."