Environmental heat stress is one of the most challenging conditions in the world which have adverse impact on the industry of poultry. Broiler chicken strains are delicate to heat stress primarily due to not having sweat glands. The current study was aimed to investigate the effects of heat stress exposed on growth performance and Bio-physiological characteristics for (Cobb, Hubbard and Arbor Acres broiler hybrids) under the summer season when environmental conditions of Egypt were revealed. A total of three hundred one day old (one hundred birds from each hybrid) at one day of age were brooding under the same conditions of water, diet consumed, breeding system, vaccines and medications used during the period birds life even slaughtering age. The three strains were randomly divided into twelve groups (three strains "Cobb, Hubbard and Arbor Acres" X two treatments "control group and heat exposed group" X two replicates X twenty five chicks).The degrees of environmental temperature and relative humidity during housing are (Environmental temperature = 32°±2 Celsius degree, Relative humidity =50±5 percentage) for control group and (Environmental temperature =40°±2 Celsius degree, Relative humidity= 20±5 percentage) for the heat stressed group. The body weight, body weight gain, edible parts of carcass (carcass, thigh, drum, breast muscles and giblets weight) and inedible parts of carcass (blood, feathers, head and legs weight) were recorded to heat stressed group and control group. Lymphatic organs such as spleen, thymus and bursa weight were measured also. The last results concluded that the Cobb strain showed the best growth performance and carcass characteristics under heat stress condition, while the Arbor Acres strain considered the best strain which didn't effected a lot in their rectal temperature such another strains with heat exposure. The Arbor Acres strain for each group (control and treated) have the highest viability. The control group and Hubbard strain showed an increase in bursa weight compared to heat exposed group and another strains. It was concluded that the Cobb strain has the best performance under heat stress comparing to the other strains broiler chicken.