Common freshwater food fishes such as the Walking Catfish (Clarias batrachus), Broadhead Catfish (Clarias macrocephalus), Climbing Perch (Anabas testudineus), and Striped Snakehead (Channa striata) are widely consumed in Southeast Asia, including Lao P.D.R. Quantitative differences in the nutritional value of these fishes were evaluated by seasons and sizes as essential nutritional information toward improving the nutritional status of each country’s residents. Size (weight and total length) and proximate composition (moisture, protein, fat, and ash contents) of the above fishes were measured quarterly over a year, with the sample fishes being separated into three size groups (large, medium, and small). Fishes of the “small” group tended to have higher moisture, protein, and ash contents than those of the “large” group across the species. In contrast, the “large” group tended to have higher fat content than those of the “small” group. In addition, seasonal fluctuations of fat content were observed in three species except the Walking Catfish, and are considered to be relevant to the reproductive cycles of each species. All the results obtained here are essential information for informing consumers of the nutritional value of the above species by species and by seasons/sizes.