Oxidative stress index (OSI) is recently used as a valid tool for providing an in depth picture about redox status and oxidative stress. The study was conducted on Fourteen multiparous, singleton bearing Osimi ewes (1.5-2 years, 35-40 kg) belonging to agriculture research station, Assiut district, A.R. Egypt during spring (as controls, N=7) and during summer (as heat stressed, N=7). Ewes were blood sampled weekly throughout the last three weeks prepartum and weekly throughout the first three weeks post-lambing for determination of blood serum lipids and oxidative stress biomarkers. Results showed increased blood serum total cholesterol, triglycerides, high density lipoprotein (HDL) and low density lipoprotein LDL (p < 0.05) before lambing and then decreased (p < 0.05) after lambing. Total Cholesterol, triglycerides and HDL was lower (p < 0.05) in summer compared to spring. In spring and summer, total peroxide (TPX) increased at the first week (p < 0.05) before and after lambing in addition to summer values were higher (p < 0.05) than spring values. In spring, total antioxidant capacity (TAC) increased at the second week (p < 0.05) after lambing, whereas a reverse trend was noticed in summer season, so that summer TAC values were lower (p < 0.05) than spring values. Pearson's Product Moment Correlation between TPX and TAC did not reveal significant indication. However, values of oxidative stress index (OSI= TPX / TAC) x100  progressively increased (p < 0.05) in summer and reached its maximal value at the first week after lambing, then steadily decreased until reached the basal data at the third week after lambing. Summer TPX values were higher (p < 0.05) than spring values. This study is the first to evaluate the degree of OS in HS ewes during the peripartum period by detection of TPX, TAC and OSI values. Our data clarified that the maximal oxidative stress occurred at the first week after parturition that was associated with an increase of pro-oxidants rather than reduction of antioxidants during spring. Instead, both the increase in pro-oxidants and the reduction of antioxidants are sharing in the oxidative stress in summer.