This study was conducted using five cytoplasmic male sterile lines (CMS) and five restorers of sunflower in a line x tester fashions at two locations over two years to determine the extent of genotype by environment (GE) interaction, combining ability, mode of gene action and to quantify the magnitude of heterosis and heritability. The traits studied were, days to emergence, days to 50% flowering, days to maturity, plant height, leaf area, stem girth, head diameter, 1000- seed weight, seeds head", oil content, seed and oil yield. The GE interaction, heritability estimates on plot and entry mean basis, general combining ability (GCA) and specific combining ability (SCA) effects were significant for all parameters. The SCA effects were of greater magnitude than GCA effects, which showed greater manifestation of non-additive gene effects. The GCA:SCA ratio also revealed predominance of non-additive gene effects. Performance of TS-18 and TS-335 among the CMS and TR-5 and TR-13 among the restorers was better for most of the traits. CMS lines TS-4 and TS-335 and restorer TR-5 and TR-6023 were better general combiners for most of the traits at both locations. Not a single cross combination showed consistent promising results for all traits, however, the cross combinations TS-17 x TR-3, TS-335 x TR-13, TS-335 x TR-3 and TS-18 x TR-5 showed higher specific combining ability effects for oil yield at Peshawar. At Mansehra TS-335 x TR-5, TS-17 x TR-6, TS-4 x TR-3 and TS-18 x TR-5 had higher specific combining ability effects for oil yield. Most of the crosses exhibited positive heterosis for all traits, however, some of the crosses also depicted negative but desirable heterosis for traits like days to emergence, days to flowering, days to maturity and plant height. High heritability estimates were observed for days to emergence, days to flowering, days to maturity, plant height, leaf area, stem girth, head diameter, 1000-seed weight, seeds head'!, oil content, seed and oil yield at both locations.