This study was conducted during three successive early summer seasons of 2016- 2018 at the experimental farm of both faculty of Agricultural, Menoufia University, Shebin El-Kom and Bahga Agricultural development company, Anshas, sharkia. The present investigation was aimed to obtain more information on combining ability and degree of heterosis for some plant and fruit characters. Four breeding lines and one cultivar of tomato (Solanum lycopersicum L.) and their ten F1's resulted from a half – diallel cross were evaluated and the component of genetic variance , combining ability and the extent of heterosis were determined for some tomato traits ,i.e., number of primary branches and leaves , early and total yield , average fruit weight and fruit firmness. The analysis of variance reflected significant differences among the studied genotypes for all the traits studied. Estimation of the magnitude of variance due to general and specific combining ability effects (s2 GCA & s2 SCA) showed that both additive and non-additive gene effects were significant and important in inheritance of all studied traits, except average fruit weight. However, the estimated genetic parameters revealed that the additive gene effects were more important and play the main role in the inheritance of early and total yield, average fruit weight and fruit firmness and could be improved by varietal breeding program. While, the non-additive genetic variance was found predominance for number of branches and leaves and improving these two traits could be occurred by heterosis breeding. The two lines MON-8 and MON-9 were considered as the best general combiner parents, since they recorded positive GCA effects for four traits.The cross combinations MON-8 x MON-9 and MON-9 x MON-15 were the best specific combinations, since they recorded significant SCA values for five and four traits.Relative heterosis (MPH%) was observed for all studied traits, except, average fruit weight, while heterobeltiosis (BPH%) was observed in some crosses for number of branches and leaves as well as early and total yield. Based on the standard heterosis (SH %) expressed by the hybrids MON-8 X MON-9 and MON-9 X MON-15 were found to be superior since significantly exceeded the commercial hybrid (control).