Warren Buffett told CNBC on Monday that Berkshire Hathaway's "third-largest business," after its insurance and railroad interests, is its stake in Apple.

"I don't think of Apple as a stock. I think of it as our third business," Buffett said on "Squawk Box."

The Oracle of Omaha's annual letter to Berkshire shareholders, released on Saturday, showed a 5.7% year-end ownership in Apple stock.

As of late last year, Berkshire owned more than 245 million shares of Apple, worth nearly $72 billion, according to a Dec. 31 filing with the government.

Apple shares are up about 80% over the last 12 months.

"It's probably the best business I know in the world. And that is a bigger commitment that we have in any business except insurance and the railroad," Buffett said.