The natural gas liquefaction process is an cryogenic system so it is very energy and cost intensive. There are several ways to reduce energy and cost consumption. One is getting optimal design and the other is getting optimal operating condition. After determined the process design, then optimize operating conditions such as temperatures, pressures and compositions. In this paper, to get optimal design in point of cost, optimization of driver selection is performed considering risk factor on C3-MR (propane pre-cooling mixed refregerant) process. Driver selection includes capital and operating cost. Capital cost could just get from venders but operating cost is very changeable. To solve driver selection more accurate, our team consider not only cost of fuels and electrics but also risk factor as the cost term. There are three scenarios to compare effects of risk factor. Risk factor depends on number of compressors which is connected to same driver. Each case is selected from mixed integer non-linear programming (MINLP) optimization results. The result is significantly different considering risk factor or not.