The purpose of this study is to analysis costs variation range through the risk factors for overseas plant projects. For this study, risk factors data from related literature review, research organization and construction company was researched and classified under each E(Engineering)P(Procurment)C(Construction) phases. The adapted research method through the questionnaire survey by plant experts was conducted for analysis of probability index and costs variation rate. Based on costs variation rate for each risk factors, the adapted research method includes application of average cost for three cases. And through Monte-Carlo Simulation, analysis of cost variation range was conducted. The results of this study are as follows: 1) The analysis of risk factors through the costs variation rate for EPC phase is Design costs variation rate -14.63%, +25.56%, Procurement costs variation rate -21.39%, +37.63%, Construction costs variation rate -31.47%, +47.07%. the analysis of costs variation range for application total average costs of three cases is to average costs 163,457 USD, maximum costs 213,710 USD, minimum costs 123,249 USD. 2) The analysis of relatively high risk factors probability index is to Design costs variation rate -7.8% +16.11%, Procurement costs variation rate -6.76%, +15.73%, Construction costs variation rate -14.72%, +25.64%. The results of costs variation range for the total average costs application of three cases is average costs 160,599 USD, maximum costs 184,635 USD, minimum costs 141,929 USD.