The aim of this study was to evaluate the effect of thermal manipulation (TM) during embryogenesis on hatchability, growth performance and thermotolerance acquisition parameters during thermal challenge (TC). Seven-hundred and fifty fertile chicken eggs were divided randomly into three groups (250 eggs each): control group was maintained at 37.8°C and 56% relative humidity (RH), TM1 was subjected to TM at 38.8°C for 6h and 65% RH during embryonic days (ED)10-18 and TM2 was subjected to TM at 38.8°C for 18 h and 65% RH during ED10-18. Hatched chicks from each treatment group were then randomly divided into two sub-treatment groups (Naive and TC). Chicks in TC groups were subjected to TC by adjusting room temperature to 41.0°C for 6h on days 3, 7, and 42 of age while naïve chicks were kept under regular conditions (25 ± 1°C and 50-60% RH). Percentage of hatched eggs was recorded and post-hatch chick performance was evaluated by recording chick body weight (BW). Chick's response to TC was evaluated by determination of body temperature (T(b)), plasma T3 and T4 levels, and muscle mRNA levels of Hsp70. There was a significant increase in muscle mRNA levels of Hsp70 during embryogenesis and during TC in post-hatch chicks. While hatchability was not adversely affected, the body weight in TM2 chicks was significantly higher at the end of the study period (42 days). Results of this study indicated a long-term enhancement of Hsp70 gene expression associated with improved thermotolerance acquisition in treated chicks without adversely affecting performance.