Atotal number of 168 hens at 22 weeks of age from two strains Silver Montazah and Matrouh (84 female each) were housed in floor pens. At 24 weeks of age, birds of each strain were randomly distributed to 4 treatments, 21 hens/ treatment (3 replicates 7 hens/ each). The first treatment was served as the control, the second treatment was subjected to heat stress, the third was supplemented with anti stress (organic selenium) at yeast selenium concentration of 0.4 mg/kg diet, and the last treatment was subjected to heat stress and anti stress. The heat exposure treatment was applied at 24 and 38 wks of age, where birds were exposed to 39± 1ºC for 4 hours / day for 4 consecutive days from 10.00A.M. to 2.00 P.M. using gas heaters. Results showed that: Body weight (BW) and growth rate (GR) of Silver Montazah strain were significantly higher than Matrouh strain. Heat stress had negative effect on BW and GR. Addition of Se-yeast as an anti stress improved BW and GR insignificantly. Body temperature (BT) and respiration rate (RR) in heat stressed hens were significantly higher than those of control. Data also illustrated that, BT and RR were higher in Silver Montazah than Matrouh strain, but this increase was not significant. Addition of anti stress as Selenium yeast exerted positive effect by reducing both body temperature as well as respiration rate under heat stress condition. Mortality rate (MR) was significantly higher in heat stressed hens compared with control and also in Silver Montazah than Matrouh strain. The anti stress reduced MR, but this reduction was not significant. Feed intake (FI) reduced in heat stressed hens compared with control, which was higher when anti stress added. Also, FI was higher in Silver Montazah than Matrouh strain. Heat stress caused a reduction in both Plasma total protein and globulin concentration, elevation in the albumin concentration, but the differences were not significant. Calcium and phosphorus in plasma also reduced under heat stress, but total cholesterol was increased. The anti stress improved this reduction keeping total cholesterol in normal level. It could be concluded that, selenium yeast as an anti stress can be recommended in both strains of developed laying hens diets to alleviate the negative effects of heat stress on laying performance under the condition of this study.