In order to investigate the heterosis and genetic relationships of different tomato lines for some qualitative characteristics, five diverse tomato lines were evaluated in a complete diallel cross design. Analysis of variance showed significant differences (P<0.01) among the genotypes for total soluble solids, total acidity, maturity index, antioxidant capacity, ascorbic acid, total flavonoids, and total phenols. The highest heterobeltiosis was observed for total phenol and total flavonoid. Results of Hayman's analysis showed that contributions of additive and non-additive effects were involved in the control of the studied traits. The genetic analysis by Hayman method showed that the attributes except the total phenol, were under the control of the dominance type of the gene action, and their low inheritance indicated the more involvement of non-additive type of gene action. Therefore, due to low heritability and lack of high selection potential, hybridization methods and heterosis phenomenon could be used in the first generation to improve these traits. In contrast, the high specific heritability of total phenol indicated that this trait is controlled by the additive gene effect. Therefore, due to the high selection potential for total phenol, it is possible to modify the population by selection the best genotypes for this trait. Overall, hybridization methods can be used to improve fruit quality traits in tomatoes.