Undefeated Roger Leonard, Sugar Ray's older brother won his 15th fight tonight with a unanimous 10-round decision over journeyman Melvin Dennis in a preliminary to the Leonard-Duran championship bout at the Superdome.

"I promise the people of the metropolitan Washington area that I will become the junior middleweight champion of the world in 1981," Leonard said after so dominating Dennis that he three times seemed ready to knock out the 10-year veteran whose 27-15-3 record includes losses to Wilfred Benitez and Vito Antefuermo.

"Dennis told me I hurt him bad in the second, eighth and ninth rounds," Leonard said, "but I just didn't have the strength to put him away. I was pacing myself more than trying for a knockout. Dennis told me I have a good shot at the title."

Leonard said his next bout will be with a top 10 contender in the junior middleweight division, "and then I want to go for the title. I'm 27 now and I want to get it pretty soon."

The championship currently is shared by Maurice Hope (the World Boxing Council champ) and Ayub Kalule (the World Boxing Association champ).

As the 10th round began tonight, Ollie Dunlap, Leonard's old mentor at the recreation center in Palmer Park, Md., shouted what he called a trade secret from his seat in Leonard's corner.

"Copenhagen, Copenhagen," Dunlap shouted again and again.

Kalule, the WBA champ, is a Ugandan who now fights out of Copenhagen.