Buffett, who owned about 81 million shares of IBM at the end of 2016, sold off about a third of that stake in the first and second quarters of 2017, he told CNBC.
"I don't value IBM the same way that I did 6 years ago when I started buying... I've revalued it somewhat downward," Buffett told CNBC. "When it got above $180 we actually sold a reasonable amount of stock."
Buffett said IBM hadn't performed the way he had expected -- or the way IBM's management had expected -- when he first started buying the shares six years ago.
"I think if you look back at what they were projecting and how they thought the business would develop I would say what they've run into is some pretty tough competitors," Buffett said. "IBM is a big strong company, but they've got big strong competitors too."
Berkshire Hathaway still owns more than 50 million shares of IBM and with shares now trading below $160, Buffett says he has stopped selling.
— For more on Buffett's moves and his reasoning behind the sale of IBM, tune into Squawk Box on CNBC at 6 am Eastern time Friday.