Under monoculture semi-intensive system, a study on cultivation of monosex Nile tilapia (Oreochromis niloticus L.) in earthen ponds (each of 6000 m²) was done.Twelve rectangular earthen ponds were used representing six  treatments, three different stocking density (SD₁ , SD₂ and SD₃  being 9, 12 and 15 fish/m³, respectivily) within each of them two fish sizes (FS₁ and FS₂ being 30 and 50 g/fish, respectivily). Two replicates were used for each treatment.  The treatment of 12 fish/m³ with 50 g/fish gave the best results of marketable size, net yield  (8820.35 kg/feddan) and highest profit index (2.97 LE). Based on the obtained results, it could be recommended that the best fish size and stocking density of monosex Nile tilapia (O. niloticus) in monoculture semi- intensive system are 50 g /fiish at 12 fish/m³ to get the optimal marketable fish size at harrvest under similar conditions to those of the present study.