This study was done to investigate of some hematological and biochemical factors of rainbow trout (Oncorhynchus mykiss) fry in west of Mazandaran in Iran. About 50 pools of blood samples from diseased fry were collected within 30 months from November 2002 till March 2005 from three hatchery farms in western part of Mazanderan province. In addition 30 pools of blood samples as control group were collected randomly from mentioned farms. Each blood samples were examined for whole blood examination and blood enzymes measurement. It consist of total leukocytes (WBC) and erythrocytes counts (RBC), hemoglobin (Hb) content, hematocrits (PCV), leukocytes differential count and blood indices such as MCV, MCH and MCHC. Also blood serums were analysed for total protein (TP) and blood enzymes. All the calculations were made using the SPSS© and t-test statistical method. In hematological findings nine parameters were revealed significant differences (P<0.05) with control group in t-test. It consisted of total WBC, Lymph, Neut, Hb and HCT, MCHC, AST, DL and total protein plasma. Also in total white blood cell count, Lymph and Neut had significant differences as compared with the control fish (P<0.05). Also blood serum components analysis revealed that only LDH and AST amount showed obvious significant differences (P<0.05). Regarding to results it could be concluded that hematological and biochemical studies could be a valuable tool for prognosis and primary diagnosis in some infectious diseases. So it could be recommended for monitoring and surveillance programs in coldwater hatchery health status in Iran.