Proximate analysis for proportions of lipids, proteins, ash, and water contents of fish tissue provides a powerful insight to obtain physiological information for the fish population being studied. In this study, the proximate compositions for the jack mackerel (Trachurus japonicus) caught in Geumo Island, Korea known as a nursery ground were examined monthly in 2013 and 2014. No significant seasonal variation was found in protein and ash contents whereas a strong seasonal variation was found in water and lipid contents. The lipid contents showed relatively higher values ranging from 3.94% to 5.75% in summer andfall which corresponds to their active feeding periods. A strong inverse relationship was found between the lipid and water contents (n=16, r=-0.859, p<0.01). The water contents was significantly (t-test, p<0.01) higher in the early period from May to August in 2014 than in 2013, which was probably caused by the smaller fork lengths on May and June in 2014. The overall lipids, proteins, ash, and water contents of jack mackerel were 3.71, 21.41, 3.65, and 68.90%, respectively, which was significantly different from the results reported previously for jack mackerel. Mainly three major factors were suggested for the difference. The first one is the different body sizes of jack mackerel since growing fish accumulate lipidsin their bodies as they feed more prey. The different methods for the proximate composition analysismight be another reason. In general, the whole fish analysis had lower water contents and higher ash contents than only fish fillet analysis. The other reason might be different sampling regions and times. A long term monitoring for the physiological status of early stage jack mackerel is expected to provide necessary information for the successful management of this fish stock.