Eleven inbred lines of silkworm, Bombyx mori L. were exposed to two main different temperature (23C and 25C) during the larval period in order to elucidate the effect of the temperature differences on their main characters under the, prevailing in Egyptial agro-climactic conditions. The results show that the temperature differences did not affect significantly the pupation ratio and cocoon shell ratio. However, they affected their larval duration to become shorter with the high temperature treatment ($27^{circ}C$) and their cocoon shell weight and cocoon yield per box of silkworm eggs to be higher in the lower temperature($23^{circ}C$). It was recommended that some inbred lines should be bred at the 23C throughout the whole larval period, particularly in being reared with high nutritional leaves in order to maximize the hybridization of cocoon yields.