Diaconu represented Romania during the Summer Olympics in Sydney in 2000. There he defeated Abdelghani Kinzi with 19 to 9, Jitender Kumar with 12 to 3, until he went in the first round KO against Jorge Gutierrez from Cuba.
After the Olympic Games 2000, Diaconu opted for a career in professional boxing. Since that time he won 24 fights, including 15 by knockout, without themselves being defeated. His first title he won in June 2005, when he won the Canadian heavyweight title by a knockout in the fifth round against Conal MacPhee. In December 2005 he won the world title in the light heavyweight of WBC after he defeated Darrin Humphrey. Furthermore, he defeated Max Heyman and captured the vacant light heavyweight title the North American Boxing Association, as well as various smaller boxing title. After his victory over Rico Hoye by technical knockout on 9 May 2007 Diaconu got the chance at a title fight for the WBC world title. On 19 April 2008, he defeated Chris Henry and was appointed interim world champion due to a violation of the real world champion Chad Dawson. In June 2008, Diaconu was finally declared a world champion right after Dawson gave up a fight. In his first title defense on 19 June 2009 against Canadian Jean Pascal, he lost his title, however, again. His opponent Pascal was at the end of the fight on all three judges forward.