Background and Aim: Silybum marianum is a plant with effective medicinal agents. The aim of this study was to evaluate the effects of Silybum marianum methanolic extract on liver function and growth parameters in common carp (Cyprinus carpio).
Methods: 180 fish with an average weight of 10.40±1.12 g were treated in a randomized design in 4 groups with 3 replications (15 fish per replication). The control had treatment without alcoholic extract and the treatments of the other three groups had 2.5, 5 and 10 g of Silybum marianum extract per kg of food, respectively. Every 15 days after anesthesia, biometrics and growth indices were assessed in all fish. Liver enzyme function was measured in the blood at the end of the study (day 90).  
Results: The highest final weight (46.20±1.34), body weight gain (36.30±1.50) and specific growth rate (1.55±0.12) in the treatment containing 10 g of alcoholic extract of Silybum marianum was observed, that had a significant difference in comparison to the control treatment (P<0.05). Also, the highest and lowest feed conversion ratios were observed in the control treatment (1.75±0.11) and in the treatment containing 10 g of extract (1.25±0.12), respectively (P<0.05). The levels of alkaline phosphatase (ALP), alanine aminotransferase (ALT) and aspartate aminotransferase (AST) enzymes in the treatment containing 10 g of extract were lower than other treatments, but no significant difference was observed between these enzymes in different treatments (P<0.05).  
Conclusion: According to the findings of the present study, consumption of alcoholic extract of Silybum marianum by common carp improved growth performance as well as the liver enzymes function.