One of the most important environmental factors that affects fish is the impact of excess heat. The consumption of herbal immuno-stimulants in aquaculture to improve immune system activity and fish tolerance to infections has become widespread. This study was conducted to determine the protective effect of garlic on fish exposed to heat stress. A total of 60 common carp fish (Cyprinus Carpio) with a mean weight of 100±10g were used. Fish were randomly divided into six groups of 10 fish in each and placed in six glass aquariums (40×40×80) cm, (G1) was exposed to heat stress at 30°C for a week. (G2) was exposed in normal water temperature 25°C for a week. (G3) was exposed to heat stress and treated with garlic by stomach tube in a dose of 1.63g/ 100g of body weight at the same time. (G4) was exposed to heat stress in the first week and then treated with garlic in the second week. (G5) was kept at 25 °C and treated with garlic. (G6) was treated with garlic in first week then exposed to heat stress in the second week. Results have shown that there were a significant increase in HB concentration, and PCV, heterophil, glucose level and total protein P<0.05 in groups exposed to heat stress. Although the results observed that the stress index had a significant decrease in the G1. This study achieved that garlic has a role to reduce the effect of heat stress on blood parameters. The best results were observed in the third and sixth groups.