The current study was carried out to evaluate the effects of thermal shock during the early rearing period on performance, carcass characteristics and some blood parameters in broiler chickens. A total of 240 Ross 308 broiler chickens was divided into 3 treatments, with 4 replicates containing 20 birds in each group. Experimental treatments were: group 1) not exposed to heat challenge (control), 2) sudden thermal shock and group and 3) gradual thermal shock. The result showed that the feed conversion ratio was significantly lower (P<0.05) in sudden thermal shock. In addition, carcass characteristics in this study were also markedly lower in the sudden thermal shock compared to the gradual thermal shock (P<0.05). Also, the thermal shock at early rearing period had no significant effect on blood parameters in this study. In conclusion, gradual thermal shock at early rearing period had a better effect on termotolerance compared to the sudden thermal shock in broiler chickens exposed to heat stress.