Skeletal deformities are important factor of evaluation of fish value commercially. Deformities of opercular are commonly observed type of fish deformation. Although these malformations in fish can be caused by culture conditions, the environmental factors are unknown. This study examined the effect of water temperature on the opercular deformity of the red spotted grouper, Epinephelus akaara. Experimental fish (TL; 7.49±0.10 cm) were respectively divided into 3 groups that were reared at 20, 24, 28℃ for 6 weeks. All specimens were photographed from the left lateral view using a Canon EOS 70D. We placed 11 landmark points for visualization the shape differences of operculum in the whole body. In order to measure the reduction of opercular, we estimated total length (TL) and shortening of the distal part (distance between landmarks 10 and 11). After 6 weeks, both growth rate and incidence of opercular shortening were high in 24 and 28℃. At 28℃, the distance of distal part of operculum was the highest as 0.36 cm and exposure to the 24℃ induced the highest growth rate during this experiment. On the other hand, both growth rate and opercular deformity were low at the lower temperature (20℃). This study shows opercular malformation as well as the growth rate of E. akaara are influenced by the high water temperature.