Buffett bought US$5bn worth of BoA preferred stock with warrants attached in 2011.
The 700mln warrants were exercisable at US$7.14 each compared to a current market price of US$24.32.
The difference is equal to a book profit of about US$12bn.
Exercising the warrants will also give Berkshire a 6.6% stake in BoA, which puts it just ahead of the Vanguard Guard that before the warrant issue held 6.6% of the bank.
Shares have soared along with its peers in recent days as big dividend hikes were promised by the bank sector as nearly all its members passed the latest US central bank stress test with ease.
Shortly after the results, BoA said it intends to raise its annual dividend by 60% to 48c per share.