This paper focuses on the morphological characteristics and microstructure of erythrocytes of five aquatic animals, in order to study the morphological diversity and evolution of erythrocytes. The blood smears of five aquatic animals, bloody clam (Tegillarca granosa, Scapharca subcrenata), Carassius auratus, Rana catesbeiana and Pelodiscus sinensis, were stained by Hemacolor stain. The surface structure and morphological characteristics of erythrocytes were examined by light microscope (LM) and scanning electronic microscope (SEM). The erythrocytes of bloody clams (Tegillarca granosa & Scapharca subcrenata) had basophil granular or/and eosinophilic granular, but none was found in vertebrates erythrocytes. The size of erythrocytes increased from the two bloody clams to Bull frog (Rana catesbeiana); while the size decreased afterwards; the variation of aspect ratio and nucleocytoplasmic ratio of erythrocytes follow this trend in a similar way. The undulate fold and depression on the erythrocytes of the two bloody clams were observed apparently, and the depression was deep and wide. The surface depression was also obvious in the erythrocytes of vertebrates, although the depression faded away gradually, and no distinct surface fold was examined. These results indicated that the changes of animal erythrocytes resulted from the evolution of animals, activities and habitats, in order to obtain the biggest gas exchange efficiency and the stability of the cells.