This study was proposed to highlight the impact of in-ovo injection with Spirulina platensis (SP) solutions in alleviating the negative effect of thermal manipulation during incubation. Also, estimate the influence of this treatment on embryonic development, hatching traits, physiological performance and the expression of gene HSP70. A total of 750 hatching eggs from a 45- wk-old of Sliver Sabahia chickens strain were numbered and weighed (51.00 ± 0.6 g). All the eggs incubated at 37.5°C (99.5 F°) and 55% relative humidity (RH) from 1 d to 11 d of incubation , then were subjected to daily thermal manipulation at 39.5°C (103.1 F°) and RH (60%) for four hours from 12^th to 18^th day of incubation. At 14^th day of incubation, 660 fertilized eggs with evidence of living embryos were randomly assigned to four groups of 165 eggs each (3 replicates of 55 eggs each). The 1^stgroup: Eggs without injection, served as (control group), The 2^nd group: Eggs injected with saline solution, served as sham control             (1 ml saline solution /egg), The 3^rd group: Eggs injected with 0.2 mg SP /egg, the 4^th group: Eggs injected with 0.4 mg SP /egg. Results conducted that Injecting eggs with SP at different concentrations (0.2 or 0.4 mg/egg) decreased egg weight loss (%) , yolk sac (%), embryonic mortality (P ≤ 0.01), pipped eggs (%) and embryonic malposition           (P ≤ 0.01) compared to control groups. While, significantly (P ≤ 0.01) improved embryonic development, hatchability percentage, chick weight and quality, most of blood parameters and the expression of gene HSP70 as compared to control groups. The best results were obtained by in ovo feeding of SP at 0.4 mg /egg. In conclusion, In ovo feeding Spirulina platensis solution with 0.4 mg/egg during thermal manipulation created new hormonal, immune systems and increasing levels of HSP70 in the embryos with more ability to withstand and challenge the temperature increase without any deleterious effect, this was reflected in improvement of hatching traits and chick quality.