The goal of this study was to assess reproductive performance, estrus signs, and lamb's birth weight in Ossimi ewes treated with organic selenium and Nano-selenium under Upper Egyptian conditions. In this experiment, a total of 30 ewes were divided into three equal groups (10 ewes /each). All of the animals were given intra-vaginal sponges (IVS) impregnated with medroxy-progesterone-acetate (MAP, 40 mg) for 14 days. Then, in the G1: (CG) control group, and the ewes in G2 (SeG) were given organic selenium 3 mg/ewe orally for 14 days from insert IVS, whereas the ewes in G3: (N-SeG) were given Nano-selenium 0.2 mg/animal orally for 14 days from insert IVS. Also, after 50±5 days of parturition (post-partum), the ewes divided randomly into 3 equal groups (10 ewes/each), and managed with the same previous treatments. The results showed that ewes given organic selenium and Nano-selenium had longer (P<0.01) in estrus duration and took a shorter (P<0.01) period to show estrus than control group in both the first service and post-partum. In addition, during the first service and post-partum, the estrus response in SeG and N-SeG were significantly higher (P<0.05) than CG. However, the intensities of estrus signs as swollen vulvar lips and redness of the vaginal membranes were clearly apparent on the ewes in SeG and N-SeG compared to the CG. Also, the body weight of lambs born from ewes treated with organic selenium and Nano-selenium increased significantly (P<0.01) compared to the control group. The body weight of the lambs in N-SeG was higher (P<0.01) after 15 days of parturition than in CG.  In conclusion, treatment with organic selenium and Nano-selenium could improve the reproductive performance, estrus signs, and lamb birth weight in ewes under Upper Egyptian climatic conditions.