The effect of diet mulberry varieties (Kanva-2, Kokuso-27, Mortiana and Balady) on some biological aspects of certain Bombyx mori L. breeds were studied under laboratory conditions. Duration period of larval stage varied significantly between silkworm breeds or between mulberry varieties. The shortest duration period of larval stage was 28.11 days for CG 16 breed when the larvae reared on Kanva-2 cultivar. While, the longest duration period was 33.58 days for JH3 reared on Mortiana. Also, the larval mortality percentage was clearly affected where the highest mortality values were recorded in CG16 breed reared on Balady mulberry cultivar (35.55%). Larval body weight was significantly increased in JH3 breed reared on kanva-2 than other mulberry cultivars. Length and weight of silk gland were significantly increased in larvae fed on kanva-2 leaves, while these parameters showed insignificant differences between the tested breeds. Larvae of EM6 fed on leaves of kanva-2 exhibited  the highest percentages of cocooning and pupation. Pupal duration and adult emergence % were affected by breed and mulberry variety. Breed JH3 fed on kanava-2 gave the highest value of pupal weight Moritiana mulberry variety and EM6 breed recorded the highest values of female longevity. Mean while, the superior longevity of male (breed JH3 and Moritiana variety) was responsed to silkworm breed and mulberry variety. Adult fecundity was decreased in female orison from larvae fed on Balady variety and CG16 breed. A positive relationship was detected between weight of pupae and fecundity. Fertility of female moth increased in CG16 larvae fed on kakuso-27 variety.