Current study was carried out to investigate the effects of adding milk thistle's extract to diet on blood lipids of broilers under heat stress. 350 one day old broilers from Ross-308 race were divided randomly into five groups and were studied in a completely randomized design with five replications and 14 broilers in each replication. Treatments were control, 5, 10, 15, and 20 ml/lit of aquatic extract in drinking water. To enforce the heat stress, temperature increased to 35-37 c o eight hours a day. At 22 and 42 days two broilers were selected from each replication randomly and blood lipids were measured. According to results, the extract reduced LDL at day 22 but didn't affect cholesterol, triglyceride and HDL. At day 42, triglycerides of 5, 10 and 20 ml groups were increased significantly in proportion to control group. LDL was also increased significantly in 5 and 20 ml groups but cholesterol and HDL were not affected. In general, using milk thistle can affect blood lipids and adjust some blood lipid parameters under heat stress dose dependently.