BACKGROUND: Aflatoxins are natural fungal toxins that weaken the immune system and damage the liver. OBJECTIVES: The effects of seeds and whole plant powder and extract of Milk thistle (MT) plant in reducing the negative effects of feeding aflatoxin (AF) on broiler chickens blood parameters and immune response were examined. METHODS: 192 one-day old chicks (Ross 308) for 35 days in a completely randomized design with six treatments, four replicates and eight birds per repetition were used. The experimental treatments included: 1) control, 2) contaminated control (CC), 3) CC + 0.5 percent of MT seed powder, 4) CC + 1 percent MT plant powder, 5) CC + 600mg/kg MT plant extract, 6) CC + 1000mg/kg MT plant extract. RESULTS: The treatments had no significant effect on plasma concentrations CHOL, HDL, LDL, ALP, LDH, AST, ImG and ImM. Feeding contaminated diet increased alanine aminotransferase enzyme compared with healthy control (P≤0.05). The addition of 0.5 percent MT seed powder, 1 percent MT plant powder and 1000mg/kg MT plant extract to the contaminated diets decreased alanine aminotransferase enzyme compared to the contaminated control (P≤0.05). Inclusion of 1 percent MT plant powder to AF infected diet significantly increased the antibody titer compared with healthy control and contaminated control (P≤0.05). CONCLUSIONS: It was concluded that compared to other treatments, 1 percent MT plant powder was more effective in reducing the negative effects of feeding AF in broiler chickens.