Effect of feeding mulberry silkworm on leaves treated with Thyroxine on the mean weights of mature larva, silk gland, pupa, fresh cocoon, and cocoon shell of mulberry silkworm, B. mori was studied under laboratory laboratory hygrothermic conditions of 25±5⁰C and 65-85% R.H. The last instar larvae which fed on mulberry leaves was dipped in Thyroxine solution at concentrations of 0.025, 0.125, 0.250, 1.250, 2.500, 5.000, and 10.000 ppm. Results indicated that Thyroxine revealed high values of the mean weight of mature larva ranged from 10.0% up to 18.9% more than untreated at concentrations of 10 and 0.025 ppm. Silk gland showed an increase of the mean weight ranged from 3.8% up to 24% at concentrations of 10.0 and 2.5 ppm. Mean weight of pupa increased by range from 3.0% up to 20.2% at concentrations of 0.025 and 2.5 ppm in comparison to untreated one. Fresh cocoon showed an increase ranged from 4% up to 19.8% at concentrations of 10 and 2.5 ppm. Cocoon shell presented also an increase ranged from 2.8% up to 17.9% at concentrations of 1.250 and 2.5 ppm in comparison to untreated one. Results indicated that the highly and significant values of the mean weight of mature larva, silk gland, pupa, fresh cocoon, and cocoon shell was gained by larvae fed on mulberry leaves treated with Thyroxine at the concentration of 2.5 ppm.