Bifi dobacterium genus has several benefi cial effectsas a probiotic bacterium. It has been demonstrated that Inulin and fat percentage of the probiotic product may modify the viability of this bacterium during storage time. The aim of this study was fi nding the best formulation of the chocolate milk regarding bacterial viability and sensory properties of the fi nal product. Four types of probiotic chocolate milk, with and without inulin in 1% fat samples, with and without inulin in 2% fat samples, were manufactured in duplicate. The count of probiotic cells during 12 days of storage in the refrigerator was assessed. In all samples, the viable cells of Bifi dobacterium lactis increased except 1% fat samples without inulin in which bacterial count decreased (not signifi cantly). The rate of change in the bacterial count was signifi cantly p<0.05 higher in the 2% fat chocolate milk both in with and without inulin samples (0.6 and 0.8) compared with 1% fat samples (0.5 and-0.4). Inulin inoculation had some effects on biochemical and organoleptic changes. After 12 days storage, the acceptability of 2% fat milk with probiotic and inuline was the highest. The results illustrated that the most signifi cant Bifi dobacterium lactis viability was in 2% fat chocolate milk. Adding inulin improved the viability of bacteria in 1% fat milk; however, ithad not any signifi cant effects on the viability of probiotic bacteria in 2% fat milk. In conclusion, 2% fat chocolate milk with inuline could be recommended to be the best group.