This experiment was conducted to study the effect of heat stress at early age on small intestine development, plasma diiodothyronine T₃ and immune response of Matrooh and Inshas chicks. Seven hundred and twenty, one day old chicks (360 Inshas chicks and 360 Matrooh chicks). The experiment included three treatments; birds from each strain were divided into three groups. The first no treated group (control T1), the birds in second group were exposed to (42^◦c - 43^◦c ± 1^◦c) for 4 hrs at three days of age (T2) while birds of the third group were exposed to the same thermal treatment of the second group but at four weeks of age (T3). After these treatments birds of (T1, T2 and T3) were raised under regular conditions. At 8 weeks of age 90 chicks from each strain (30 bird from each group) were subjected to heat stress (42^◦c - 43^◦c ± 1^◦c)   for 4 hrs. blood samples were taken from (5 chicks)  before and after this heat stress. At the end of experiment (18 weeks of age) 5 birds from each treatment groups were slaughtered and the jejunum was removed for histological procedure. The important results obtained are : 1) Early heat stress acclimated bird showed significant effect on small intestine morphology, there was significant difference in villus ( height, width, volume ) than control. 2) Early heat acclimated birds (at 3 days) had the lowest plasma T₃ content compare to control and (T3). 3) Early heat acclimated birds (3 d or 4 weeks of age) had significantly higher antibody titer than control. Conclusively, the obtained results showed that morphological change observed in the small intestine illustrated one possible mechanism for the loss of bird production induced by heat stress. It appears, therefore, that heat exposure in early age can improve thermo tolerance at maturity improving the ability to reduce T₃ concentration and consequence reduces heat production.