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), TM 1 was subjected to TM at 38.8°C for 6h and 65% RH during embryonic days (ED)10–18 and TM 2 was subjected to TM at 38.8°C for 18h 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 T 3 and T 4 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 TM 2 chicks was significantly higher at the end of the study period (42days). 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.