During larviculture of the orange-spotted grouper, Epinephelus coioides, cannibalism is a frequent phenomenon. The aim of this study was to investigate the prey size selection of the cannibalistic orange-spotted grouper fry. The results showed that when offered prey of three sizes, the cannibal preferred the smaller prey to the larger ones although it was capable of engulfing the larger individuals. Size selection should be related to an optimal foraging strategy by the grouper. An increase in the energy in pursuit and handling time for larger prey would exceed the benefits gained from consumption of the prey. Eating smaller prey can reduce the risk of failing or the prey escaping. Thus, grouper can efficiently gain energy through cannibalistic acts against smaller prey.