Abstract: Due to the high risks in oil and gas development projects，economic evaluation and risk assessment is essentially important. Based on the analysis of main risk factors affecting oil and gas economic evaluation，such as oil and gas production，price，operation cost，IRR，and investment costs，taking the development case of a certain gas reservoir as example and adopting Monte Carlo Analysis，this paper makes aquantitative analysis on economic evaluation and its risk. The conclusion is that compared with the conversional NPV method，Monte Carlo Analysis can make an integrated consideration of various risk factors. It could not only obtain the assessment result but also the probability distribution，and the risk result is more reliable，more practical and can better reflect the equitability of the evaluation results. The authors suggest that Monte Carlo Analysis should be enhanced in economic evaluation of oil and gas projects in order to improve the accuracy of policy decision.

Key words: oil and gas projects, economic evaluation, risk factor, risk analysis, Monte Carlo analysis