Nikolic said that if the European Union adhered to rules it would not admit Croatia as its member until Zagreb resolved that issue.
"I wouldn't mind anyone saying that Croatia should resolve the issue of the Serbs, especially of Serb-owned property, and I often discussed it in Brussels. I don't know why the EU raised Croatia two or three notches above us on that path (to EU membership)," Nikolic said in a programme broadcast by the Belgrade-based Prva Srpska TV network.
Nikolic said he did not see any problem with Croatia other than the issue of the Croatian Serb refugees and the restitution of their property.
According to the latest opinion polls, Nikolic is the main rival to the recent President of Serbia and the presidential candidate of the ruling Democratic Party (DS), Boris Tadic, and Nikolic's SNS is seen as the main rival to the coalition led by Tadic's DS.
Serbia will vote in presidential, parliamentary, regional and local elections on May 6.