Bioavailability of calcium from milk and other calcium sources has been an important issue of studies over recent years. The present study compared the bioavailability of calcium from basal diet containing cow milk (CM), yoghurt from cow milk (YCM), yoghurt   from cow milk plus either Bifidobacterium lactis Bb-12 (YCMBb-12) or Bifidobacterium longum Bb-46 (YCMBb-46), non- fermented soymilk (NFSM), soy-yoghurt resulted from fermentation with either B.lactis Bb-12 (YSMBb-12) or B.longum Bb-46 (YSMBb-46) and basal diet alone (control). The obtained results indicated that there were no significant differences in food intake, body weight gain and food efficiency among the different groups of rats fed experimental diets in comparison to control. During the experimental period, feeding diets containing probiotics significantly increased serum calcium concentration and significantly reduced the faecal calcium (mg / day) compared to the control diet. The apparent absorption % of calcium was significantly increased too. The caecum acidity (pH), cecal carboxylic acids and bifidobacterial count were also increased in the groups of rats fed probiotic. These cecal parameters have a positive correlation with calcium bioavailability. The levels of femoral and tibial strength were also significantly increased by the probiotic diet. It is worthy to note that diet containing YCMBb-12 was the most effective and in general probiotic cow milk yoghurts were much better than those of soymilk yoghurt in enhancing the bioavailability of calcium. Thus the results in the present study proved the beneficial effects of cow milk yoghurts and soymilk yoghurts containing probiotics in enhancing calcium bioavailability and subsequently lowering loss of body calcium.