Three types of granulocytes, neutrophil, eosinophil and small eosinophilic granulocyte, were observed in peripheral blood of Siberian sturgeon Acipenser baerii. The neutrophil had three types of granules, eosinophilic granule (αG； light blue) in the cytoplasm. These granules were round or oval, but the maximum diameter was different each other (αG, ≦0.3 μm; βG, ≦1.5 μm; γG, ≦0.4 μm). The nucleus of the neutrophil showed polymorphism, round, oval, kidney and multi-lobes (to five lobes). Alkaline phosphatase (AlP), acid phosphatase (AcP), β-glucronidase (β-Glu) and some esterases were detected as positive granules. Peroxidase (PO) activity was not observed in the neutrophil. Eosinophil had polymorphic nucleus and two types of eosinophilic granules (EG), large EG (LEG) and small EG (SEG). Both granules were round or oval and the diameter of LEG (≦1.1 μm) was larger than SEG (≦0.4 μm). The former was stained orange, the latter showed brick color. In eosinophil, AcP, β-Glu and some esterases were positive, but AlP and PO were negative. Small eosinophilic granulocyte was small sized cell with condensed nucleus and some eosinophilic granules. The cell was round to oval, and had high nuclear to cytoplasmic ratio. The morphology and staining characteristics of the granules were similar to LEG of eosinophil. No enzyme activities were detected in the granulocyte. Phagocytosis against zymosan particles was observed in only neutrophil.