"NATO is the key military alliance without which the Kosovo problem will not be solved," he said at a press conference in Bucharest with his Romanian counterpart Ioan Micrcea Pascu.
Tadic said "the situation would be different" if Serbia and Montenegro were a member of the so-called Partnership for Peace.
The partnership aims to reinforce stability and reduce the risk of conflict through defense cooperation. It has been joined by 30 countries throughout eastern Europe since the collapse of communism.
NATO currently leads leads a 20,000-strong international peacekeeping force (KFOR) in Kosovo.
Kosovo, a province in southern Serbia, has been a UN protectorate since NATO bombing in 1999 forced Serbian troops to relinquish control and call off a brutal crackdown on the majority Albanian population.
Clashes between ethnic Albanians and Serbs last month resulted in 19 dead and 900 injured in the worst violence the province has seen since it came under UN and NATO control.
The defense minister also called for the decentralization of Kosovo in order to ensure political, economic and cultural autonomy for Serbian enclaves and to provide "safe conditions" for Serbians and non-Albanians.
"Stabilizing the situation means, before anything else, creating favorable conditions for the return of Serbians who had been forced out of the province and the reconstruction of churches and Orthodox monasteries" destroyed in last month's violence, Tadic said.
Romania's defense minister also pleaded for Serbia and Montenegro to join the Partnership for Peace, calling it the "first step" toward stabilizing Kosovo.
"We have noticed a great willingness in Belgrade to engage in dialogue with the international community and the Albanian party," Pascu added.