Recently two breeding programs were carried out for isolation of new parental inbred lines in Iran. This study was undertaken in order to estimate the combining ability effects and heterosis of these lines. For this purpose an $8 imes8$ diallel cross analysis including eight inbred lines of silkworm with four lines from each program were studied for their five quantitative traits. The results indicated that reciprocal effects appeared insignificant for most of traits. Japanese lines of 111 and 113 were best combiners for shell weight, cocoon weight and shell percentage traits. With respect to pupation rate, Chinese line 110 was the best and according to general combining ability (GCA) and specific combining ability (SCA) variances, it is superior in specific combinations. Hybrid $109 imes110$ showed better characters for both productive and viability traits. Hybrids with high heterosis had high SCA too.