Frank Tate, a 2-to-1 underdog, knocked down Michael Olajide Jr. twice en route to winning a unanimous decision in their International Boxing Federation middleweight championship fight yesterday at Caesars Palace in Las Vegas.

In gaining the title, which had been vacant, Tate became the fourth member of the 1984 U.S. Olympic team to win a world professional championship.

With about a minute left in the 11th round, Tate landed a jab and followed with a right to the side of the jaw that dropped Olajide. After struggling up at the count of five, it looked as if Olajide might fall again as referee Richard Steele continued the mandatory eight-count. He stayed upright, however, and weathered a two-fisted barrage in the final minute of the round.

In the opening moments of the 12th round, Olajide missed with a left hook and Tate came over it with a right hand that landed above and to the right of Olajide's left eye. Olajide went down and took a nine-count on his knee.

But Tate was too arm-weary to follow up and was content to box in the last three rounds.

Judge Jerry Roth scored it 147-136, judge Chuck Giampa saw it 146-135 and judge Bill Graham 148-134, all for Tate.

The 23-year-old Tate, 160, a native of Detroit who lives in Houston, has a 21-0 record with 12 knockouts. Olajide, 157 1/2, also 23, is 23-1 with 16 knockouts. Olajide, who was born in England, raised in Vancouver and lives in New York, was ranked No. 1 by the IBF.