This research investigates electric power system expansion considering climate change. Now and in the future, climate change is and will be affecting new power plant investment decisions and electricity generation system in more uncertain ways. The power system needs to be more reliable, cost-effective and environmentally friendly when exposed to higher temperature, less precipitation and more intense and frequent extreme events. Traditional modeling is not sufficient to incorporate climate change effects and uncertainty. In this paper, the uncertainty of climate change is taken into consideration. The input variables and system parameters that are impacted by climate are identified and quantified. Different climate scenarios are used with corresponding input profile to represent all the possible outcomes of a future climate instead of continuous probability distribution. A robust electric power Generation Expansion Planning (GEP) optimization model minimizing the expected total cost under all scenarios is formulated and solved to find the optimal result. Therefore, a good compromise solution that is nearly optimal for all scenarios is chosen to avoid the possible risk brought by a poor decision that is only optimal for one particular scenario.