Background & Aim: Marshmallow (Althaea officinalis) is one of the most popular medicinal plants used in traditional medicine with antibacterial and antioxidant properties. This study was conducted to evaluate the effects of marshmallow extract (Althaea officinalis L.) administration on blood cells and biochemical parameters of carp liver. Experimental: 150 carp (weighing 37.65 ± 4.40 g) were fed with diets containing 0.0 (control diet), 2.5, 5, and 10 g marshmallow extract for 60 days. Then, red blood cell indices (RBC), hemoglobin (Hb), hematocrit (Hct), mean corpuscular volume (MCV), mean corpuscular hemoglobin concentration (MCH), white blood cells differential count and biochemical parameters of liver such as aspartate aminotransferase (AST), alanine amino transferase (ALT), lactate dehydrogenase (LDH), and alkaline phosphatase (ALP) were measured on days 30 and 60 of the experiment. Results & Discussion: On days 30 and 60 of the experiment, red blood cell indices (RBC), hemoglobin (Hb), hematocrit (Hct), mean corpuscular volume (MCV), mean corpuscular hemoglobin concentration (MCH), white blood cells differential count and biochemical parameters of liver such as aspartate aminotransferase (AST), alanine amino transferase (ALT), lactate dehydrogenase (LDH), and alkaline phosphatase (ALP) were measured. On day 60, no significant difference was found (p>0.05) in RBC, Hb, Htc, MCV, MCH count, number of neutrophils, basophils, and eosophils in fish fed with different concentrations of marshmallow extract. Administration of marshmallow extract (5 g) significantly increased (p<0.05) WBC and lymphocytes on day 60. On the other hand, administration with 5 and 10 g of marshmallow extract decreased monocytes (p<0.05) on day 30. The significant increase was observed in AST, ALT, ALP and LDH levels in liver of fish fed with 10 g of marshmallow extract may be attributed to cytotoxicity, however 2.5 and 5 g of extract did not have adverse effects on liver enzymes. Industrial and practical recommendations: According to the results, it was concluded that preclinical administration of certain concentrations of marshmallow extract (2.5 and 5 g) was beneficial for carp. پرونده مقاله