Since 2012, Korean government has implemented the renewable portfolio standard (RPS) program to promote renewable electricity production. Renewable energy certificate (REC) market has been operated under the RPS program to provide flexibility for mandatory electricity power companies when they cannot fill the gap between their own renewable electricity production and required renewable electricity by the RPS. However, high fluctuation of the REC spot market price has affected negatively investment on renewable power plants. In this context, this study investigates main factors that significantly affect the REC spot market prices based on the monthly data between 2012 and 2021 in Korea by using the Autoregressive Distributed Lag (ARDL) with error correction (EC) estimation method. We found that the accumulated excessive supply of RECs and the volume of RECs sold in the REC spot market are main determinants that affect the REC spot market price. In addition, revision of fixed price-long term contract system and higher REC weights have negative impact on REC spot market prices.