This paper uses an approach for evaluating the effectiveness and risks of investment projects based on fuzzy logic for the formalization of uncertainty. In this work considered some input variables like net present value (NPV), internal rate of return (IRR), project payback period (PB), accounting rate of return (ARR), investment return index (PI). The value of the output variable determines the probability of acceptance of the project, its effectiveness and risks in the implementation process. As linguistic terms, it is proposed to use triangular and trapezoidal fuzzy numbers. The authors use a fuzzy logic approach for evaluating the effectiveness of investment projects based on production rules. This allows us to simplify the process of evaluating investment projects, especially in cases where the influencing factors (evaluation criteria) are not connected by a single mathematical formalization, their number is large enough for calculations, estimates of projects by criteria can be not only quantitative but also qualitative. In this case, it is enough to create a knowledge base from experts in this field. Described and use the Mamdani algorithm of fuzzy logic inference when solving the current problem of evaluating investment projects under uncertainty.