The present study is conducted to investigate the supplementary effect of soy flour and spirulina as protein sources added to mulberry leaves with different concentrations (2.5, 5 & 10%w/v) on the biological, economical and physiological characters of three hybrids of silkworm, Bombyx mori; two imported hybrids (Bulgarian and Chinese) and the local Egyptian one (was used as a control or reference value). The schedule of application was applied since the beginning of the fifth larval instar till end up to spinning (one diet/day). Most of the studied treatments concentrations revealed a significant enhancement in the silkworm characters under investigation; a significant increase was exhibited in larval and pupal weight, cocooning percentage and the effective rate of rearing of the imported hybrids compared with the local one. Especially, larvae reared on the concentration 5% of soy flour then spirulina with the concentration 2.5%. Furthermore, the same concentrations of protein sources increased significantly the cocoon and silk filament characters. Moreover, the total protein content and protease activity of larval haemolymph exhibited a significant increase owing to rearing on mulberry leaves fortified with the same concentrations of protein additives respectively. Conclusively, soy flour as a high protein content nutritional additive with the tested concentrations is superior and better than spirulina. As well, silkworm larvae of the imported hybrids (Bulgarian and Chinese) are manifested more susceptive positively to protein nutritional additives than local hybrid. So, it could be prescribed to the breeding programs to achieve high quantity and quality of cocoon yield for silk production.