The histological,surface and internal fine structures of gill filaments of Cheilinus undulatus were investigated using light,scanning and transmission electron microscopy.The results showed that there were 3 pairs of holobranch,1 pair of hemibranch and 1 pair of pseudobranchia.The comb primary filaments lined on the gill arches closely.Many secondary gill lamellae inlaid on two sides of each filament,and arranged each other closely.The blood system of filament was composed of afferent artery,efferent artery and capillary vessel nets.Various features such as indentations,micropits and crevices were present on non-respiratory surface of filaments.The respiratory surface of filaments was folded.The filaments were covered by epithelial cells,which had different surface features in different areas.It had regular fingerprint shape microridges in non-respiratory surface,no microridge but folded surface in respiratory area and irregular microridges in the transition region.There were many holes on the non-respiratory surface.Chloride cells and mucous cells were observed and many mitochondria and excretion vesicles distributed in chloride cells.Secondary filaments were consisted of epithelial cells,basement membranes,endothelial cells and pillar cells.These structures formed the water-blood barrier.The close relationship between the structure of fish gill and its function was discussed.