This research was carried out to evaluate the performance of eight Egyptian cotton genotypes and their F1 hybrids under Upper Egypt heat conditions using Line x Tester analysis, during 2016 and 2017 seasons. In addition, to determine the combining ability, heterosis and gene action which control yielding ability and fiber traits. Eight Egyptian cotton genotypes and 15 crosses were evaluated at Shandaweel Agricultural Research Station, Sohag governorate. Analysis of variance indicated that genotypes, parents, crosses and parents vs. crosses were significant or highly significant for all the studied traits, except lint percentage in parents and parents vs. crosses, which were insignificant. The mean squares due to lines or tester (G.C.A.) were significant or highly significant for most of the studied traits. Line x Tester (S.C.A.) main squares was highly significant for most yield traits, while insignificant Line x Tester (S.C.A.) mean squares were found for all fiber quality traits. Regarding mean performance and heterosis, the varieties Giza 90, Giza 95 and Giza 86 were the best parents in yielding ability and gave high yielding crosses under heat conditions, while Giza 45 and Giza 92 were the good parents to produce the best fiber quality crosses. The results of heterosis also showed that seven crosses had positive and highly significant heterosis based on mid-parents in seed and lint cotton yield /plant and number of bolls/plant i.e., (Giza 80 x Giza 90), (Giza 86 x Giza 90), (Giza 86 x Giza 95), (Giza 87 x Giza 90), (Giza 45 x (Giza 90 x Australian)), and (Giza 92 x Giza 90), while the cross (Giza 92 x Giza 95) had better yield and fiber traits. The line Giza 86 was the best combiner for seed and lint cotton yield/plant, number of bolls/plant and seed index, while lines Giza 45 and Giza 92 were the best combiners for fiber fineness, fiber strength and fiber length. The tester Giza 90 was the best combiner for seed cotton yield/plant and lint cotton yield/plant. Four crosses exhibited positive and significant values of specific combining ability (S.C.A.) effects for seed cotton yield/plant, lint cotton yield/plant, lint percentage and number of bolls/plant. The non-additive of genetic variance was larger than additive genetic variance in all yielding ability traits and additive genetic variance was higher than dominance variance for all fiber quality traits. Broad sense heritability (Hb%) was higher than narrow sense heritability (Hn%) for all traits and high heritability estimates in narrow sense were found for all fiber traits.