Environmental concerns, growing electricity demand and energy security are key drivers for planning of electricity generation capacity. The present study is based on an energy system model in Iran during 2015-2050. Special focus is laid on the long-term development of electricity generation technologies in different conditions, considering two demand groups: 1- business as usual (BAU), and 2- energy efficiency scenario. MESSAGE as a bottom-up energy supply optimization model is employed to assess the appropriate electricity generation options to meet the final demand in these scenarios. The results indicate that during the planning horizon, total installed capacity grows at annual rate of 3.6% and 2.4% in BAU and energy efficiency scenarios, respectively. The cumulative investment cost in energy efficiency scenario is US$75 billion (in 2014prices) lower than the BAU scenario.