Abstract: 【Objective】 The aim of this study was to analyze the microscopic and ultrastructural characteristics of peripheral blood cells of Gymnocypris eckloni, Platypharodon extremus, Schizopygopsis pylzovi and Gymnodiptychus pachycheilus, enrich the hematological parameters of the 4 rare fish species in Yellow River, and provide physiological data for the protection of rare fish resources in Yellow River. 【Method】 ArcGIS Desktop 10.5 was used to draw a schematic diagram of the sampling sites, gill nets and ground traps were used for fishing. After the 4 rare fish species in Yellow River were anesthetized with MS-222, blood was collected from the tail vein with a 1 mL syringe treated with sodium heparin anticoagulation, and microstructural and ultrastructural samples were prepared. Well-stained peripheral blood smears were selected, and the proportion of each type of cells was counted. ImageJ was used to measure the long and short diameters of different types of blood cells and the long and short diameters of the cell nucleus. 【Result】 The results of cell microstructure and ultrastructure observation showed that in addition to mature erythrocytes, immature erythrocytes and mitotic erythrocytes were also observed in the peripheral blood of the 4 rare fish species in Yellow River. No basophils were observed in the microstructure of peripheral blood cells of the 4 rare fish species in Yellow River, but a small number of basophils were observed in the ultrastructure. The special granules in the eosinophils of Gymnodiptychus pachycheilus were composed of dense crystal core and matrix, which was different from the other 3 fish species. In the peripheral blood of the 4 rare fish species in Yellow River, the surfaces of granulocytes and monocytes were all distributed with pseudopodia, and the mitochondria and rough endoplasmic reticulum in the cytoplasm of monocytes were well developed. Primary lysosomes and secondary lysosomes were also observed in the monocytes of Gymnodiptychus pachycheilus. The results of cell proportion statistics and size measurement showed that the proportion of lymphocytes in the peripheral blood of the 4 rare fish species in Yellow River was the highest, followed by neutrophils, and the proportion of eosinophils and monocytes was relatively low. There was no significant difference in the long diameter of erythrocytes in the peripheral blood of the 4 rare fish species in Yellow River（P＞0.05）. The short diameter of erythrocytes in Gymnocypris eckloni was significantly larger than that in the other 3 fish species（P＜0.05, the same below）. The long and short diameters of the erythrocyte nucleus of Gymnodiptychus pachycheilus were significantly larger than those in the other 3 fish species. The long and short diameters of eosinophils in Platypharodon extremus were significantly larger than those in Gymnodiptychus pachycheilus, and the long and short diameters of the nucleus were significantly larger than those in Schizopygopsis pylzovi. The short diameter of small lymphocytes in Gymnocypris eckloni was significantly larger than that in Platypharodon extremus, and the long and short diameters of the nucleus were significantly larger than those in Schizopygopsis pylzovi. 【Conclusion】 The peripheral blood cell composition of the 4 rare fish species in Yellow River is similar. Erythrocytes, thrombocytes, neutrophils, eosinophils, monocytes and lymphocytes can be seen in the microstructure, and basophils can also be seen in the ultrastructure. The mature erythrocytes of the 4 rare fish species in Yellow River are similar in size, which may be related to the fact that they live in the same water environment. Lymphocytes account for the highest proportion in the peripheral blood of the 4 rare fish species in Yellow River, followed by neutrophils. The abundant lymphocytes may be related to their morphological adaptability.