This experiment was conducted for a period of 5 weeks in order to study the effects of Clostridium butyricum ID on the growing performance and the changes in microbial flora of the feces for growing pigs. One hundred and forty four head of three way crossbred (L x LW x D) growing pigs weighing approximately 14㎏ to 15㎏ were employed into four treatment groups consisted of control, Q butyricum ID 0.01%, Cl. butyricum ID 0.02% and Cl. butyricum ID 0.03% for the feeding trial, metabolic trial and microbial assay. The results obtained were summarized as follows. 1. It was found that growing pigs fed Cl. butyricum ID were better than control in daily weight gain, but no significant difference was found. Although significant differences were not found among treatments, pigs fed Cl. butyricum ID consumed more feed than control group. Feed efficiency was significantly improved (p $lt; 0.05) by the addition of 0.03% Cl. butyricum ID compared to other treatments. It should also be noted that, as the level of Cl, butyricum ID increased, feed efficiency was improved steadily. 2. Digestibility of experimental diets was slightly, but nonsignificantly increased by the Cl. butyricum ID compared to control. The addition of Cl. butyricum ID at the level of 0.02% resulted in significant increase in nitrogen retention rate (p $lt; 0.05), but no differences were found among treatment groups fed Cl. butyricum ID. 3. The supplementation of Cl. butyricum ID to the growing pig diets caused sharp change in microbial flora in the feces after 3 weeks. It was observed that Cl. butyricum and Lactobacilli were increased, but Staphylococci and Coliforms reduced greatly by the addition of Cl. butyricum ID. From observations mentioned above, it could be concluded that the supplementation of Cl, butyricum ID for growing pig diets could improve the weight gain and feed efficiency through the increase of digestibility and nitrogen retention, and by prevention of diseases probable with the injurious microbial flora in feces. Therefore, it may be suggested that Cl. butyricum ID can be used as an effective feed additive for growth promotion of growing pigs.