He was publicly against it but practiced it his entire life. He even had a 14 year old sex slave named Sally Hemming who he fathered children with and then put his own children into slavery.

Now Americaphilic apologists will tell you that he was firmly opposed to it but in reality he had a lot of assets and was accustomed to a very privileged lifestyle. Freeing his slaves would have required him to forgo a lot of luxury and he was simply not willing to do so.

Jefferson was a constant opponent of slavery throughout his whole life. He believed that slavery contradicted the laws of nature, which decreed that everyone had a right to personal liberty. During the American Revolution, he drafted a Virginia law that prohibited the importation of enslaved Africans. In 1784, he even suggested an ordinance that would ban slavery in the Northwest territories. Even though Jefferson was against slavery, he believed that blacks were racially inferior and believed they should be removed from the U.S. because he thought that white Americans and enslaved blacks were "two separate nations" who could not live together peacefully. So while Jefferson was all for abolishing slavery, he was a bit of a racist and wanted the slaves to go back to the West Indies or Africa.

Source: http://www.monticello.org/site/plantation-and-slavery/thomas-jefferson-and-slavery