On June 1, 1996, Dieppa won his first regional professional championship when he defeated Pablo Tiznado in a card that took place in Miami, Florida. In this fight he won the Fecarbox Flyweight championship, a regional title sanctioned by the World Boxing Council. During this timeframe Dieppa was trained by Félix Trinidad Sr. and was under contract with Don King. Dieppa won three, non-title bouts in a row, including a unanimous decision in eight rounds against Kenny Berrios on June 7, 1997. His next fight was against Carlos Murillo and was held on February 13, 1998, at the University of New Mexico in Albuquerque. The judges awarded Murillo a split decision victory, marking Dieppa's first professional defeat. On October 3, 1998, he returned to activity in a fight against Orlando Malone. Dieppa won the fight by unanimous decision. He participated in three fights between 1999 and early 2000, winning all of them by knockout.