In the future, renewable energy (RE) generation systems will be constructed and interconnected to the power system by the non-utility entities such as generation companies and residential customers. Therefore, the generation expansion planning (GEP) should consider the effects of RE generations. The purpose of this paper is to discuss the best GEP from economic, supply reliability and environmental aspects considering penetration of RE generations. For this purpose, this paper proposes a model of utility's GEP process considering the RE generations. The proposed method is based on the stochastic dynamic programming (SDP) in which the long-term uncertainties are modeled by the geometric Brownian motion (GBM) and the binomial lattice process. Furthermore, the variation of RE generation output due to the weather condition (short-term uncertainty) is also considered in the proposed method by means of the net load curve.