Abstract: Background and Objective: Manipulating the incubation temperature, particularly during the late stage of embryonic development can strongly affect the performance of hatching chicks at later ages. This study was conducted to evaluate the influences of thermal conditioning during embryonic development and at an early age post-hatching on growth performance, metabolic hormones and the skeletal muscle profile of broiler chicks. Materials and Methods: A total of 510 hatching broiler breeder eggs were obtained from Cobb-500 broiler breeder flock at 48 weeks of age. At embryonic day 16 (E16) post candling and infertile exclusion, the eggs were randomly divided into two main groups, each of 240 eggs. The first group was set at optimal incubation conditions (T0), while the second (T1) was exposed to thermal conditioning (TC) at 39.5±0.1°C for 5 h/day at E16, E17 and E18. On day 4 post-hatching the (T0 and T1) groups were each randomly sub-divided into two sub-groups. One sub-group of each main group was exposed in a controlled chamber to TC at 38.0±0.1°C and 42.0% RH for 5 h/day through the 4th, 5th and 6th day of age, while the other sub-group was kept under optimal brooding conditions, resulting in four final sub-groups, T0T0 (optimal incubation and brooding conditions), T1T0 (embryonic TC), T0T1 (post hatch TC) and T1T1 (pre- and post-hatch TC), all were reared up to the age of 42 days. Data on growth performance, metabolic hormone concentrations and histomorphometric measurements of the myocytes of the pectoral muscles were analyzed with a one-way analysis of variance. Results: The results showed that all the TC groups, especially T1T1 had higher (p<0.01) growth and carcass performance than the non TC group (T0T0). With pre- and postnatal TC, whether separately or in combination, the IGF-I level was increased, T4 was reduced, while the T3 level did not change significantly at 42 days. In addition, the major pectoralis muscles of T1T0 and T1T1 chicks possessed greater number and histometric measurements of myocytes compared to those of the other groups. Conclusion: Pre- and postnatal TC both separately or in combination could enhance the productive performance and breast muscle development of broiler chicks.