This experiment was conducted to examine the effect of multi-strain PROBIOTIC (Primalac) on PERFORMANCE, immune responses and CECAL MICROFLORA in BROILER chickens under CYCLIC HEAT STRESS condition. A total of 96 one-d-old mixed sex BROILER chicks (Ross 308) were weighed and randomly allocated to two treatment groups, each with 4 replicate pens of 12 chicks. The dietary treatments were basal diet (control) and control+ 0.9 g Primalac/kg diet. Body weights of BROILER s were determined at d 1, 21 and 42, feed intake was de-termined at the same periods, and feed conversion ratio was calculated accordingly. The populations of Lactobacilli spp and coliforms were enumerated in the cecum. Antibody titers against Newcastle, Bronchi-tis, and Gumboro were measured as immune responses at 28 d of age. As a result of this study, use of pro-biotic significantly (P< 0.05) increased BROILER PERFORMANCE by enhancing body weight, daily feed intake and decreasing the feed conversion ratio. The Lactobacilli spp. population in birds supplemented with PROBIOTIC significantly was higher and coliforms population was lower than control groups at 42 d of age (P< 0.05). Also administration of the PROBIOTIC appeared to improve the antibody responses to Newcastle disease virus and Bronchitis, Gumboro disease vaccination. In conclusion, the results indicate that supplementing BROILER reared under heat stress condition with 0.9 g Primalac/kg diet could induce favorable influences on per-formance, immune responses and CECAL MICROFLORA.