This thesis has studied benefits and costs of the Pak Mun Dam project using both quantitative and qualitative approaches. For the quantitative analysis, EGATs economic analysis are used as the principal references. EGATs feasibility studies are carefully examined, theoretical and empirical comments are made, and the recalculation analysis is conducted following the comments. Three items of benefits and three items of costs are incorporated in the recalculation analysis, i.e., electricity benefit, fishery benefit, tourism benefit, Pak Mun project capital cost, economic cost of evacuation, and Pak Mun Project operation & maintenance cost. Results of the recalculation show that the Pak Mun project has the internal rate of return of 7.45% (Compared to 17.36% in EGATs study). At 12% discount rate, the net present value and B/C ratio of the project are -834.28 million baht and 0.87, respectively (compared to 2254.34 million baht and 1.31 in EGATs study). This means that the Pak Mun project should be regarded as economically unfeasible. The reduction in values of these evaluating criteria is caused mainly by three reasons. Firstly, the reduction of electricity benefit due to the change of the alternative power plant choice from gas turbine power plant with diesel fuel to the coal-fired thermal with FGD imported-coal fuel. Secondly, the reduction of electricity benefit due to the reduction of size of the alternative power plant from 150 MW installation capacity to 76.53 MW (75 MW dependable capacity). Finally, the exclusion of irrigation benefits and costs due to the independence of the upstream irrigation project from the existence of the Pak Mun reservoir. However, results of the sensitivity analysis have shown that the most sensitive variable in this projects cost-benefit calculation is the assumption of alternative power plant choice. If it is the case that Thailand is lacking of the peaking power plant capacity rather than the base-load power plant capacity, it will be more appropriate to use the gas turbine with diesel oil fuel as the alternative power plant than the coal-fired power plant with FGD and imported-coal fuel. The effect of this assumption is so great that it can raise the value of the project IRR up to 13.20% and thus reverse the result of the recalculation to be economically feasible. In the qualitative part, descriptive discussions of some important consequences that are not included in the quantitative calculation are conducted. It is found that the most two important probable impacts of the project are the loss in fish biodiversity and the risk of blood fluke (Schistosomiasis). The problem is that, up to present, neither of the magnitudes of these two impacts can be clearly indicated. Moreover, it should also be noted that the exclusion of these environmental impacts from the quantitative analysis implies that the quantitative study has underestimated the actual social cost of the project.