For evaluation the stability and potency of anthrax vaccine prepared in Razi Vaccine and Serum Research Institute in Iran, samples of different batches of vaccine were kept at 4-8 °C (refrigerator), 20-25 °C (room temperature) and 37 °C (incubator).The viable spores/ml of vaccines were determined using plate–counting method before and after holding at different temperatures monthly. Vaccine potency and duration of immunity conferred by the vaccine in animals were determined by challenging each group of vaccinated animals with virulent strain of Bacillus anthracis at 6 months intervals up to 24 months post vaccination. The results showed that the spores in vaccine remained viable at the recommended level for up to 3 years at 4 °C, 2 years at room temperature and 2 months at 37 °C. All the vaccinated animal groups resisted and survived against challenge 6, 12, 18 and 24 months post vaccination. So, it is concluded that the anthrax vaccine could be stable up to 3 years at 4-8 °C and the immunity conferred by vaccine in sheep and goats lasts for 24 months post vaccination.