Abstract: Using electron microscopy, we studied the ultrastructure of peripheral blood cells of Cromileptes altivelis. Six types of cells (erythrocyte, granulocyte, monocyte, lymphocyte, thrombocyte and plasma cell) were detected under the electron microscope. Erythrocyte is round or oval with visible mitochondria and a few vesicles. According to the size, shape of particles and form of nucleus, granulocytes can be divided into 4 types: Ⅰ-type granulocyte, Ⅱ-type granulocyte, Ⅲ-type granulocyte and Ⅳ-type granulocyte. Monocyte is round with smooth surface and vacuoles, occasionally holding out pseudopodia. Lymphocyte has a few organelles and little cytoplasm which only forms a thin layer around the nucleolus. Thrombocyte is rich in heterochromatins which distribute zonally along the nuclear membrane. There are many vacuoles at various sizes and a small amount of mitochondria. Rough endoplasmic reticulum distributes hierarchically around the nucleolus in plasma cell. In addition, erythrocytes could be seen to be phagocytized by eosinophils and thrombocytes gathered in groups.