Renewable Portfolio Standard (RPS) requires a certain percentage of electricity production from renewable. The market power in the process of RPS implementation always affects the renewable energy certificate/credit (REC) market and electricity market. This paper develops dominant firm-competitive fringe models that account for market power. It shows that market power could have significant impacts on the REC and power prices. Through empirical research, this paper gets the effects of market power on REC and power prices with different RPS requirements increase in different dominant firm situation. This paper indicates that lower REC price could lead to an underinvestment in renewable in the long run. Meanwhile, higher REC price could distort investment incentives, resulting in inefficient resource allocation for power generation in the long run. Therefore, regulatory agencies need to carefully oversee the market performance to ensure a healthy development of renewable industries under the RPS policies.