This paper proposes a new simulation tool for generation expansion planning (GEP) considering variety of uncertainties and investment options. The proposed simulation tool can treat plural uncertainties such as demand growth or fuel price in future, and select adequate investment options such as construction, decommission and reconstruction of generation plant. The proposed simulation tool can also treat the electricity market under the deregulated environment. The influences for several elements - plural uncertainties, investments and competitor's behavior on GEP are discussed through case studies using the proposed simulation tool.