Freshwater fish in the Nong Han reservoir provide a source of food and livelihood for local people who live in this unique aquatic ecosystem. The diversity, in terms of species of freshwater fish, needs to be assessed regularly in order to understand the available fisheries resource. The objectives of this study were to determine the species diversity of freshwater fish in the littoral and limnetic zones. Gill nets (about 2,000 m length and 2 m deep) in different mesh sizes were used to collect freshwater fish in March and April 2011. We found that currently there are at least 23 species (12 families) of freshwater fish in the Nong Han reservoir. We also found that the littoral zone shows a higher species richness and number of individuals than the limnetic zone. Cyprinidae was the major family, with Puntius brevis and Cyclocheilichthys apogon being the most common species. The results of this study provide valuable information, particularly on the biodiversity, and are relevant to the conservation and restoration of the fisheries in this large body of water.