The study presented here was conducted to investigate the effects of genisteinisoflavone on performance, immunological responses, ileal digestibility of crude protein, ether extract, ash, acid insoluble ash, and gut micrifloraof Ross broiler chick fed on different dietary crude protein (CP) levels. A total of 576 one-day-old Ross 308 broiler chicks were randomly allotted to the 4 replicates of each 12 dietary treatments Dietary treatments consisted of a 3×4 factorial arrangement of treatments including 3 different dietary CP levels control, and 1 and 2 percentage points lower) and 4 supplemental genistein levels (0, 20, 80 and 320 mg/kg of diet) which fed during a 42 days feeding trial. Performance parameters were recorded at biweekly intervals. The sera samples were obtained from two chicks per pen after immunization against Newcastle (NDV) and infectious bursal (IBD) disease viruses.Result show that although dietary treatments had no effect on feed intake, however, decreasing dietary CP level by 2 percentage points reduced weight gain, resulting in a marked decrease in feed efficiency. On the other hand, dietary supplementation of genistein caused the improvement in weight gain and feed conversion efficiency. supplementalgenistein was more effective in diets containing lowerCPlevels caused significant dietary CP × genistein interactions for body weight gain and feed conversion ratio. Although dietary CP level had no marked effect on ileal microbial populations, however, supplemental genistein (especially at the levels of 80 and 320 mg/kg of diet) Reduced total bacteria and Escherichia coli counts. Dietary inclusion of 20 and 80 mg/kg genistein caused an increase in antibody production titer against NDV and tended to improve IBD antibody titer. From the presents results, it can be concluded that dietary CP level can be reduced by at least one percentage point in response to dietary supplementation of genistein in broiler chicks. Keywords: Broiler chicks, Genistein, Different protein levels, Humoral immunity, Isoflavonoids, Leukocyte subpopulations, Ileal digestibility.