Most countries have implemented renewable energy expansion policies in order to respond to climate change, strengthen energy security, and create job opportunities. This paper aims to identify conditions for successful renewable expansion by renewable portfolio standards (RPS) which is planned to be adopted by Korea in 2012, while carrying out a comparative case study for countries with RPS. For this purpose, five cases including the state of California, the state of Texas, Sweden, UK and Japan were selected and their historical achievement were analyzed in terms of expansion rates of RES-E, observance of mandatory quotas and cost-effectiveness. As a result, different from theoretical legitimacy, RPS was successful in only three cases such as the state of California, the State of Texas, and Sweden, other two cases such as the UK and Japan were not successful. This paper found the fact that sucess and failure of RPS depend on the scale and characteristics of renewables of individual country and institutional design of RPS. Consequently, RPS may achieve its policy goal only when these conditions are satisfied. This implies that Korea which does not have large-scale renewable sources needs to reconsider introduction of RPS or to complement RPS policy design for efficient renewable energy expansion.