This research was conducted to investigate the effect of stocking density and supplementaryrnfeeding on growth performance and yield of Oreochromis niloticus in cage culture in WonjirnReservoir. The treatments had stocking densities of 25 (25F), 50 (50F), 75 (75F) withrnsupplementary feed and 25 (25N), 50 (50N), 75 (75N) fish per m3cage without supplementaryrnfeed. The treatments with supplementary feed were in duplicates and the controls were single.rnJuveniles with average weight of 43.70 Â± 0.19, 43.12 Â± 0.5, 43.38 Â± 1.14, 43.97 Â± 0.36, 43.51 Â±rn0.33 and 43.98 Â± 1.18 gm were stocked in 25F, 50F, 75F, 25N, 50N and 75N treatments,rnrespectively. The fish were fed 3% of their body weight twice per day using feeding trays forrn100 days. The results indicated that growth performance of O. niloticus was density dependent.rnThe final mean weight of O. niloticus in 25F was 175.83Â±0.68 gm and the mean daily weightrngain was 1.36 gm, whereas in treatments (50F, 75F) and controls (25N, 50N and 75N), finalrnmean weight was 154.92Â±0.82, 146.70Â±0.15, 86.67Â±0.51, 73.64Â±0.26, 72.62Â±0.46 gm andrnmean daily weight gains were 1.06, 0.77, 0.55, 0.28 and 0.27 gm, respectively. Moreover, feedrnconversion and condition of fish were affected by stocking density and supplementary feeding.rnThe apparent feed conversion ratio (1.45 â€“ 4.45) was significantly affected by stocking densityrn(P 0.05). In this experiment, it can be concluded that thernmost effective stocking densities for O. niloticus cultured were at 25 fish / m3 cage for largerrnsize fish demand in short period and 75 fish/m3 for higher gross production withrnsupplementary feed. Physical, chemical and biological parameters were the same among cagesrnor between cages and the open water in the present experiment. Thus, all O. niloticus hadrnsimilar culture environment and equal access to natural feeds.rnKey words / phrases: Cage culture, Growth performance, Wonji Reservoir, Nile tilapia, Stockingrndensity, Supplementary feeding.