Abstract. A total of 720 newly-hatched Arbor Acres broilers were selected to investigate the effects of fructooligosaccharide (FOS) and B. subtilis on performance and microbial gut concentration in broilers. The result indicated that dietary supplementation with FOS or Bacillus in comparison to an antibiotic had selective effects on increasing caecal concentration of beneficial bacteria such as Lactobacillus and decreasing concentration of harmful bacteria such as E. Coli and Salmonella. However, supplemental aureomycin had non-selective effects on caecal microflora which inhibited all bacteria. Moreover, the combination of FOS and B. subtilis had much better improvement on caecal micro-ecosystem for broiler than they were used solely. Dietary addition of FOS or B. subtilis improved performance of broilers to the levels of those fed the diet with aureomycin and the combination of FOS and bacillus had better effects on reducing diarrhea rate and promoting growth than they were used solely.