In the present study pathogenicity of Yersinia ruckeri have been studied in common carp, Cyprinus carpio, fingerlings. In this regard, 120 common carp fingerlings with average weight of 13 ± 2 divided into 10 aquariums (12 fish per aquaria) and were reared for 28 for acclimatization. In this study one control and 4 treatment groups (with two replicates for each one) were considered. Fish of treatment groups were challenged with Yersinia ruckeri  via intra peritoneal injections at dosage of 5.4×10⁸, 5.4×10⁷ , 5.4×10⁶, 5.4×10⁵cells/fish. Control fish just received sterile normal saline (0.9% NaCl) via intra peritoneal injection. According to the results mean lethal concentration (LC₅₀) of this bacterium after 7 days and until 21 days post challenge calculated as 2.8 ×10⁸ cell/fish. Congestion in internal organs especially in kidney and accumulation of bloody fluid in the abdominal cavity were the most clinical signs in sick fish. Gill histopathology was characterized by hyperplasia and epithelial lifting in secondary lamella, kidney histopathology was characterized by congestion and bleeding in interstitial tissue, accumulation of fluid in tubular lumens and tubule necrosis and so in the liver congestion and hepatocyte necrosis were recorded as the most histological signs