Abstract-This article tends to examine the roles of middle powers in regional and multilateral affairs, in order to develop a more comprehensive understanding of the currently debatable 'middle power' concept in international relations. Middle powers are states with moderate influence and strategic importance as a part of the international system, as they are able to implement policies to distinguish their identity without relying on decisions made by great powers. The writer argues that based on their most prominent power resource and foreign policy practices, middle powers could be classified into three categories, i.e. Enforcers, Assemblers, and Advocators. Firstly based on Realism, middle power Enforcers possess adequate hard power resources, high economic development, and are capable of deploying their forces to ascertain regional geostrategic domination; for instance China and Russia. Secondly in line with Neo-Liberalism, middle power Assemblers implement diplomatic behavior to establish regional institution, engage in long term mutual consultation to enhance regional trust and initiate peaceful dispute settlement between regional member states; such as Brazil, India, Indonesia, Mexico, South Africa and Turkey. Finally relevant to Constructivism, middle power Advocators have prominent soft power capabilities, launch branded foreign policies on low-political issues, and play leadership role on ad hoc coalitions in multilateral forums; for example Australia, Canada and South Korea. This article concludes that 'middle power' is not merely an academic concept, but by understanding their middle power resources, states would be able to develop practical foreign policies in regional and multilateral affairs.