Cyprinus carpio L. is one of the most important freshwater fish and has been intensively cultivated. However, microbial infection become restricting component that can cause economic loss in carp production and the use of commercial antibiotics for therapy produces adverse side effects. This study was conducted to evaluate the potential of Ocimum sanctum L. crude extract on haematology of Cyprinus carpio against A. hydrophila infection. For this purpose, fish that have been infected were immersed in different doses of Ocimum sanctum L. crude extract (50, 150, 250, 350 ppm) and compared with the control group. After 1 week of immersion, total erythrocytes, total leukocytes, and deferential leukocytes (neutrophils, monocytes and lymphocytes) were recorded. The result showed that significant increase of total erythrocytes was obtained at the concentration of 350 ppm Ocimum sanctum L. crude extract. Furthermore, decreasing of the total leukocytes about 55.06×103 to 25.77×103, also decreasing deferential leukocytes (monocytes 40%, neutrophils 31% and lymphocytes 49% from the positive control). These results suggested that 350 ppm of Ocimum sanctum L. crude extract is beneficial to enhance the haematological status of common carp against A. hydrophila.