This study was performed to detect the efficacy of probiotic (PROBAC plus) as anti-stress on behavior and welfare of heat-stressed broilers (32-35oc) from age 22d to 42d. One-hundred twenty chick were divided into 4 groups. The first group was control (basal diet), second was broilers reared at high temperature (basal diet), third was broilers reared at high temperature (basal diet + Probiotic) and fourth was probiotic group (basal diet + probiotic). Behavioral patterns of broilers were recorded from week 4 till the end of the experiment. At last day of experiment, 10 birds from each group were slaughtered to collect blood samples. Blood films were stained for measuring heterophils / lymphocytes (H / L) ratio. Serum cortisol was measured. The results showed that feeding, comfort behaviors, and movement activities of heat-stressed broilers were decreased while the drinking frequency was increased. Heat-stressed birds showed higher H / L ratio and cortisol concentration. Addition of probiotic either to diets of birds reared with or without stress improved some behaviors of birds and decreased H / L ratio and cortisol hormone. These results suggest that feeding of probiotic can be a useful tool to counteract the adverse effects of heat stress in broiler industry.