Turkey electricity sector has been facing a dramatic increase in demand in the last decade. The CO2 emission reduction targets and limiting the use of imported fossil fuels put a strain on the fuel mixture planning of electricity sector. In this study, least cost electricity generation based on various fuel mixtures with and without external costs are determined while achieving an optimal solution for emission reduction targets or keeping the fuel share as it was in the base year (business as usual, BAU). The optimization is developed between 2016 and 2035 using ANSWER-TIMES energy planning simulation program taking 2015 as the base year. The results show that by internalizing external costs renewable energy power plants are promoted in long term. The fossil fueled power plants are primarily preferred to meet the demand when the external costs of electricity generation are not considered during optimization. The scenario results also presented that the total discounted costs increase by 4% for CO2 mitigation scenario relative to BAU scenario when external costs are not considered in optimization. However, total discounted system costs are increased by about 30% for both CO2 mitigation and BAU scenarios when external costs are considered in optimization. The scenario results also presented that the CO2 emissions can be reduced by about 30% when optimizations are performed considering the external costs and emission targets. (C) 2017 The Authors. Published by Elsevier Ltd.