In terms of investment decision making, it is vital for the decision makers, to be able to select the investment project that is the most profitable. Therefore, various models are established including the net present value (NPV) and the internal rate of return (IRR) for the investors to compare different investment projects. In this paper, studies by scholars focusing on the advantages and disadvantages, and the modifications as well as applications of the NPV and IRR models are summarized and presented, along with further prediction of future market situations. Studies have also suggested that the prediction of cost of capital has to be carefully taken into consideration in NPV, and the pitfalls of IRR when encountering non-conventional cash flows which multiple numbers of IRR may be attained. In addition, further studies have shown that the feasibility of NPV and IRR are limited, due to the non-conventional cash flow, thus several modified versions of NPV and IRR were constructed including Max-NPV, Decouple NPV, Modified IRR, and Average IRR. Moreover, this paper analyses the improvements among the amendments upon the two models, by eliminating the problems of inconsistencies of NPV and IRR; case studies are also discussed to clarify the practical uses of the modified versions. Despite the coherent results obtained from the calculations, analysis of individual investment projects should still be done, in order to work out the optimal decision.