Rationalism vs. constructivism is an ongoing great debate in the International Relations Theory (IRT), the debate topics are on: 1) materials matter or ideas matter in shaping agency's behaviors. 2) international institution's effects. 3) identity formation and international norms compliance. 4) logic of consequences or appropriateness mode of decision-making. 5) explanation or understanding in international studies. This great debate will bring about to reorganize the IRT itself, an empirical research's dominance in IRT, neo-perspectives which integrate the rationalism and constructivism, and the convergence of constructivism and postmodernism.