Background and aim of the work:Antioxidants can reduce damage produced by low doses of radiation on living cells.This study was designed to investigate the effects of fermented soy milk (FSM) and low dose of gamma radiation on carcinogenic effect of N-methyl-N-nitrosourea (MNU). Materials and methods:Female rats were divided into 8 groups: group (1): control, group (2): injected with MNU, group (3): whole body exposed to low dose of gamma radiation (0.5 Gy), group (4): given FSM orally, group(5): given FSM and MNU, group (6): received MNU and exposed to gamma radiation, , group (7): given FSM, MNU and exposed to gamma radiation . Results:Fermented soy milk exerted significant, ameliorative effect on glutathione peroxidase, superoxide dismutase and catalase activities, lipid peroxidation and TNF-α levelin ratsinjected with MNU. Combined treatment of FSM and low dose of gamma radiation markedly elevated GSH level, ameliorated MNU effect on cell cycle phases Go/1, S, G2/M and induce apoptosis via activation of caspase-3. Conclusion:FSM consumption with exposure to low doses of gamma radiationreduced carcinogenesis and oxidative stress effects induced by MNU in the mammary tissues. Key words: fermented soy, N-methyl-N-nitrosourea, mammary gland, cell cycle, TNF- α, gamma radiation, antioxidant state.