The purpose of this study is to analyze the Probability Costs variation ranges of the cost items from the risk factor model for construction phase of overseas gas plant projects. For this purpose, the risk factors were first examined through literature reviews and the main risk factors were selected according to its level of significance and occurrence rate through interviews with professionals. The percentage changes in the total construction cost for each main risk factors were computed and applied to the example case from the past. Monte-Carlo Simulation was employed on these results to analyze the probabilistic variation range. The results obtained from the above procedure are as follows. First, there were a total of ten risk factors including the schedule changes for materials procurement, the prolonged construction schedule, the modifications in design, and the changes in materials costs. Second, the variation ranges of the cost items affected by the above risk factors are as follows. The labor costs varied from 7,702 to 7,815 million KRW; the subcontracting costs varied from 83,688 to 85,495 million KRW; the contingency allowances varied from 7,330 to 7,468 million KRW.