Feeding habits of Hippocampus japonicus (4.1~10.4cm SL) collected from am eelgrass bed in Dongdae Bay, Korea were studied. H. japonicus was a carnivore which consumed mainly gammarid amphipods. Its diets also included a small amount ofcaprellid amphipods, tanaids, copepods, mysids, and eelgrass. Gammarid amphipods were the most important prey organisms to the diet of all size classes of H. japonicus despite smaller H. japonicus (< 4.5cm SL) fed copepods. H. japonicus fed on larger sizes of prey with increasing size. The dietary breadth of H. japonicus were decreased with increasing fish size.