To study the pathogenicity of Yersinia ruckeri to rainbow trout (Oncorhynchus mykiss), this paper established a pathological model of enteric redmouth disease (ERM) and developed the corresponding scoring system of the clinical signs and histopathology. 43 healthy O. mykiss with the average weight of 12 g were randomly divided into 5 groups: three experimental groups (n=30) , control group (n=10)and sentinel group (n=3). The fish in three experimental groups were intraperitoneally injected with Y. ruckeri at a concentration of 2.0×10⁶, 2.0×10⁷ and 2.0×10⁸ CFU/mL, respectively. The clinical signs and pathological changes of intestine, liver, spleen and kidney in the infected fish were determined. Bacteriological test was also used to evaluate the effect of the infection model. The challenged fish presented different degrees of death in 72 h and showed some clinical signs such as red mouth, anal swelling, different degrees of congestion in fins (pectoral, pelvic, anal fins), obvious blood spots in lateral line and abdomen. Histopathology indicated that the infiltration, hyperemia and hemorrhage of cells in the liver, spleen, kidney and intestine were observed. Degeneration and necrosis were found in the liver cells, intestinal epithelial cells, renal tubular epithelial cells and other parenchymal cells. In spleen, the number of lymphocytes decreased and the red blood cells heaped up to death. The results showed that the best disease model could be constructed by Y. ruckeri at a concentration of 2.0×10⁷ CFU/mL for O. mykiss and the clinical signs were significant with little differences and the disease was in a slow progress. The pathological model of the enteric red mouth disease was successfully established by intraperitoneal injection of 0.1 mL Y. ruckeri at a concentration of 2.0×10⁷ CFU/mL for O. mykiss juveniles with the body weight of about 12 g.