Total of 360 chicken (90 chicken from each strain) were randomly assigned in to three treatment with three replicates and each replicate contain 30 chicks. The first treatment served as a control group where birds reared under ambient temperature, birds in second treatment were exposed to heat stress at 37oC for 4 hours and birds in third treatment were exposed to 39oC for 2 hours from day old to 14 days and 29 to 42 days of age, apart from exposure period birds were reared at ambient temperature. The results showed a significantly (P<0.05) decreased body weight, body weight gain and feed consumption at 6th week of age in thermal conditioned birds, irrespective of strains. However, at the end of 12th week broiler strains (Nandanam Broiler-3 and Naked Neck) had significantly (P<0.05) increased body weight, body weight gain and feed consumption. It was concluded that during thermal conditioning body weight was decreased, whereas at later days compensatory growth occurred to improve the weight at marketing age.