Heterosis and gene action on oil%, and yield traits were determined in a 6x6 diallel cross of cotton without reciprocals and their F₂ generation to define and select an efficient and prospective material for immediate use in hybridization programs to improve seed yield of cotton in Egypt. Parents, F₁ and F₂ were evaluated in a randomized complete block design (RCBD) with three replicates for yield traits in 2021 season. High significant mean squares for genotype, parents and crosses were showed for the studied traits in both generations. Significant heterosis in F₁ generation was obtained for all studied traits. The desirable heterosis of relative to better parent varied from 5.36 to 25.29, 5.92 to 33, 2.3 to 5.89 and 1.14 to 7.38 in F1 generation for  seed cotton yield, lint yield, seed index and oil%, respectively. The cross P2xP3 was the best cross for all studied traits heterosis. Mean squares for general (GCA) and specific (SCA) combining ability were significant for all studied traits. MS (GCA)/ MS (SCA) ratios displays the relative importance of additive and additive by additive gene action effects in their inheritance for seed cotton yield, lint yield , seed index and lint percentage in F1 generation and seed cotton yield and lint yield in F2 generation.  P1 exhibited significant desirable  effect among all the tested parents for lint percentage, lint index and oil% in F₁ and seed cotton yield , lint yield and oil % in F₂. The cross P2xP3 showed significantly desirable SCA effects for most studied traits.