The current study amid to study the effect of in ovo injection by SeNPs on chicks weight, hatchability, thyroid hormones (T3 and T4) and histological parameters at the first day of the experiment of improved Baladi chicken. A total number of 525 fertile improved Baladi eggs (Saso with Golden Montazah) with an average weight of 51.30g±0.81 were used. Before incubation, eggs were randomly divided into seven groups. The first group was non-injected eggs, considered as control(T1), the second group was injected into air sac with sterile water with volume of 0.1 ml/egg (T2), the third group was only punctured (T3) while the fourth (T4), fifth (T5) , sixth (T6) and seventh (T7) groups were injected into air sac with 0.1 ml sterile water containing 0.05, 0.075, 0.1 and 0.2 µg SeNPs respectively . The eggs were incubated at normal condition of incubation. Results obtained could be summarized as follows: 1- Hatching weight was significantly (P≤0.05) increased in treated groups compared to the control groups. 2- Hatchability of fertile eggs was significantly (P≤0.05) decreased by ovo injection with SeNPs as compared to the control. 3- Plasma T3 and T4 at the first day of age was significantly (P≤0.05) increased by ovo injection with SeNPs as compared to the control. 4- At hatch, there is no obvious histological difference in the studied organs such as liver, thymus, bursa and kidney except the thyroid gland in between the seven studied groups.