This work aimed to study the effects of Nano- selenium (Nano-Se) supplementation and in- ovo injection on the growth performance and physiological responses of post-hatch chicks reared under hot conditions.  A total number of 60 laying hens and 8 cocks Doki-4 strain (74 weeks old and average body weight of female and male 1663.75±34.9 g and 2383.33±147.0, respectively) were randomly divide into two equal groups (30 hens and 4 cocks of each). The first group considered as parents of treatment 1 (Tr1) and treatment 2 (Tr2). The second group considered as parents of treatment 3 (Tr3) and treatment 4 (Tr4). After four weeks of start experiment, 300 fertile eggs collected from two groups (150 eggs / group), the fertile eggs and post-hatch chicks treated as following; Tr1, parents and post-hatch chicks fed on basal diet plus in- ovo injection with 0.5 ml saline solution /egg (control treatment). Tr2, parents and post-hatch chicks fed on basal diet plus in- ovo injection with 0.5 ml saline solution (containing 0.2 µg Nano –Se) /egg. Tr3, parents fed with basal diet supplemented with 0.3 mg Nano –Se /kg feed and post-hatch chicks fed on basal diet plus in- ovo injection with 0.5 ml saline solution (containing 0.2 µg Nano –Se)/ egg. Tr4, parents and post-hatch chicks fed on basal diet supplemented with 0.3 mg Nano –Se /kg feed plus in- ovo injection with 0.5 ml saline solution (containing 0.2 µg Nano –Se) / egg. The results showed that, hemoglobin and mean corpuscular volume significantly (P<0.05) increased in Tr2 and Tr3 in compared to Tr1 (control). Heterophils / lymphocyte ratio and hatchability % significantly (P<0.05) improved in Tr3 and Tr4 in compared to Tr1. Immunoglobulin M significantly (P<0.05) increased in Tr2 and Tr3 in compared to Tr1 and Tr4, rectal temperature significantly (P<0.05) decreased in Tr4 in compared with Tr1, from one day to 6 weeks of age from hatch, Tr3 and Tr4 show better performance in body weight gain, feed intake and feed conversion when compared with Tr1 and Tr2. In conclusion, supplementation the diets of parents and post-hatch chicks with 0.3 mg Nano-Se / kg feed plus in-ovo injection with 0.2 µg Nano-Se /egg enhance growth performance, hematological parameters and immunological responses of post-hatch chicks reared under hot conditions.