Tank irrigation was considered by the Thai Government to be the most appropriate type of irrigation project for much of Northeast Thailand. Tank irrigation has many desirable properties. First, it is appropriate for the undulating topography and small tributaries of the region. Second, each project requires a relatively small amount of capital and time for construction. Third, the number of projects can be spread throughout the region to reach as many people and as many communities as possible. Fourth, it does not require highly sophisticated technology or highly skilled labor. These properties fit well with the economic, social and political conditions of the nation as a whole and of the Northeast region in particular. Tank irrigation started out to serve political purposes more than economic objectives. In an attempt to get the program going rapidly, more emphasis was put on increasing the number of tanks rather than on full completion of the tanks already started and on maintaining the system already constructed. Many tanks were left with unfinished distribution systems, and many are in a poor state of maintenance. The present study is an ex post economic evaluation of five tanks, namely: Huai Sathot, Huai Kaeng, Huai Aeng, Nong Ya Ma and Nong Krathum. These tanks are different in size (2.5 - 36.0 mcm), topography (steep to flat), and performance (satisfactory to marginal). A field survey of farmers was conducted in 1979. The results from the analysis shows that all five tanks are economically acceptable in spite of a wide range of performances. The share of agricultural benefits of total benefits is higher in large tanks than in small tanks. However, size of benefit from domestic uses is quite significant (50 percent or more of total benefit) in all tanks due mainly to benefit from fish culture. An analysis of the distribution of project net benefit shows that project net benefits are more equally distributed among the irrigated farms than the distribution of income from rice production among the non-irrigated farms. The benefits of the tank projects appeared to favor the medium and small farmers over the large farmers.