The fugitive General Ante Gotovina must surrender to the Hague war crimes tribunal, said Rehn.
He also said that Europe wanted the negotiations with Croatia to start, but added that the Union had done its homework.
"The rest is up to Croatia," Rehn said.
He underlined that the extradition of Gotovina was very important for the credibility of the UN court as well as for the further tracking down of Bosnian Serb war criminals Radovan Karadzic and Ratko Mladic.
According to the Hague tribunal, Croatia is not cooperating fully. Zagreb's cooperation with The Hague is one of the conditions for Croatia's EU entry talks to start.
Rehn said the European Commission was in contact with the office of chief prosecutor Carla del Ponte.
He voiced confidence del Ponte would change her assessments of Croatia's cooperation if there was cause to do so.
"The best probability, of course, is to locate Gotovina and hand him over to The Hague," he said.
"Then everything would be clear and serious negotiations on Croatia's EU entry could start," Rehn concluded.