This experiment was carried out to evaluate the possible effects of the probiotic Saccharomyces cerevisiae dietary supplement on body weight and some rumen parameters in lambs. Totally fifteen native growing male lambs (with initial age and body weight of 4.88±0.91 months and 23.21±2.75 kg respectively) were randomly divided to three equal groups (five lambs/each). The first group (C) fed the basal ration without any supplementation, the second group (Y) fed the basal ration supplemented with 0.5 g of live yeast (levucell SC 20.)® daily for each lamb and the third group (YC) fed the basal ration supplemented with 7 g of yeast culture (Diamond V XP™)® daily for each lamb. Rumen liqour samples were collected before supplementation of probiotics and then every 15 days until the day 90th. Rumen liquor pH, ammonia-nitrogen, protozoa counts and total volatile fatty acids concentrations (TVFAs) were estimated. Further, body weight and daily weight gain were calculated. Rumen parameters were significantly affected by treatments (P ≤ 0.05). Also, body weights and weight gain were significantly increased (P ≤ 0.05) in both supplemented groups (Y and YC).