Thirty six entries of flax (Linum usitatissmum, L.) involving eight parental genotypes and their twenty eight hybrids were evaluated for straw yield and its related traits in the F1 during (2015/2016) and F2 populations during (2016/2017) under early (F2D1) and late (F2D2) sowing dates (two environments) in the Agricultural Research Station, Faculty of Agriculture, Ain Shams Univ., Shalakan, Kalubia Governorate. Mean squares due to genotypes, parents and crosses were highly significant for straw yield, plant height, technical stem length, fiber yield/plant, fiber percentage, fiber length and fiber fineness in the F1 and F2 generations (at early and late sowing dates), indicating that parental genotypes as well as their F1 and F2 generations exhibited reasonable degree of variability for all studied traits. Highly significant variations mean performance were found for parental genotypes and hybrids for straw yield/plant and its related traits in the F1, F2D1and F2D2, indicating presence of wide genetic variability among studied genotypes. The highest mean values were recorded under all studied environments, for straw yield/plant, fiber yield/plant and fiber fineness were found by S. 402/1 genotype, while Sakha 5 gave the highest mean values for plant height, technical stem length and fiber length.  General and specific combining ability mean squares were highly significant for all studied traits in F1, F2D1 and F2D2 with the exception of a few cases, indicating the importance of both additive and non-additive gene effects in the expression of  straw yield and its components. The additive effects were more important than non-additive effects under all studied environments for straw yield/plant (except at F2D2), plant height (except at F1), fiber percentage and fineness. On the other hand, the non- additive effects were more effective than additive effects for technical stem length, fiber yield/plant and fiber length. Results showed that the parents; Giza 11and Giza 12 for straw yield/plant, Sakha 5 for plant height and fiber length, Sakha 6 for fiber yield/plant, Jowhar  for fiber percentage and S. 402/1, Sakha 2, Giza 12 and Sakha 5 for fiber fineness, these parents appeared to be the best general combiners for these traits. Some of the crosses exhibited highly significant and positive SCA effects included high x high and high x low general combiner parents, suggesting that the breeding procedure which utilize both additive and non-additive genetic variances would be more useful for improvement of straw yield and its components of flax.