"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.
Berkshire counts insurance giant Geico Auto Insurance and North American railroad BNSF as wholly owned subsidiaries.
Buffett said he wished he bought Apple long before he did: "I should have appreciated it earlier."
The billionaire investor revealed in a May 2018 CNBC interview that Berkshire Hathaway had upped its Apple stake by an additional 75 million shares, adding to its already huge stake at the time of 165.3 million shares.
Berkshire Hathaway, through one of Buffett's lieutenants, bought its first 10 million Apple shares in May 2016.
Buffett, a self-admitted Luddite, resisted using an iPhone, in favor of his flip phone, even as Berkshire doubled down on Apple stock. But on Monday, he told CNBC that he finally gave in and switched to an iPhone.
"My flip phone is gone," he said. "I've been getting several of them" over the years, including from Apple CEO Tim Cook.