This paper discusses the efficiency of the index of Net Present Value(NPV)when it is used to evaluate investment project under uncertain conditions and points out that the effects rising from risks should not be neglected in this condition.According to the statistical features of investment projects,the paper put forward the calculation of this index under the uncertain conditions.Finally,an example is provided in the paper to further illustrate the use of the way.So by following the suggested way in this paper,we can make the investment decision more reasonable.