Economies of scale is an effective way of reducing cost in enterprises. However， in current electric generation project investment decision， the value of the option for generation units investment of scales are overlooked. Considering fuel cost variation， this article studies generation units with different scales and energy sources， sets objective function as minimizing cash flow of future cost， considers value of flexibility option， and builds mathematical model generation investment decision-making， then introduces decision criteria based on real option， and improves net present value（NPV） decision criteria.