Six parental cultivars and /or lines of bread wheat were used in a half diallel cross at Gemmeiza Agricultural Research Station, at Egypt during  two growing seasons of, 2008/2009 and 2009/2010 to study combining ability and heterosis and their interactions under two nitrogen fertilizer levels (35 kg N/ faddan (low N)and 70 kg N/ faddan ) for days to heading, grain filling period, plant height, number of spikes / plant, number of grains / spike, 1000- grain weight and grain yield / plant. Nitrogen fertilizer levels mean squares were found to be highly significant for all studied traits. Being higher at normal nitrogen than those of low nitrogen fertilizer level. Genotypes, parents and the resultant crosses mean squares were found to be highly significant for all traits studied at the two nitrogen levels and their combined except for grain filling period at low nitrogen level of parents. Parent vs. crosses mean squares as an indication to average heterosis overall crosses were found to be highly significant at the two different nitrogen levels and their combined for grain filling period, plant height, 1000-grain weight and grain yield/plant. The interaction of nitrogen levels with genotypes, and crosses were found to be significant for all traits studied except days to heading. General combining ability ( GCA ) and specific combining ability ( SCA ) mean squares were found  to be highly significant for all traits studied at the different nitrogen fertilizer levels and their combined. This would indicate the importance of both additive and non- additive genetic variance in the inheritance of all traits studied. The interactions of nitrogen fertilizer levels with both types of combining abilities were found to be significant for all traits studied except days to heading and number of grains/spike. The cultivar P₆ proved to be a good combiner for days to heading , plant height, number of grains / spike and 1000- grain weight at the two nitrogen fertilizer levels and their combined. The hybrid combination ( P₄ x P₅ ) showed highly significant desirable SCA effects for plant height, 1000-grain weight and grain yield / plant. As for grain yield / plant, four hybrid combinations ( P₂ x P₄ ),( P₂ x P₅ ), ( P₂x P₆) and ( P₃ x P₄ ) showed highly significant desirable heterosis which varied from 9.407% to 71.542 % relative to their better parents at both low and normal nitrogen fertilizer levels and their combined.