The influence of oral intake of an antibiotic (Norfloxacin®) through mulberry leaf at two concentrations (50 and 100 ppm) on the feed conversion efficiency parameters of silkworm hybrids, viz. bivoltine (CSR2 × CSR4) and crossbreed (BL67 × CSR106), was studied. Feed consumption data showed that the food consumption of antibiotic-treated batches was on par with the control. However, nutritional indices parameters such as digesta, approximate digestibility and reference ratio were significantly higher in the antibiotic-treated batches. Similarly, the nutritional efficiency parameters such as efficiency of ingested food into larvae, cocoon and shell were significantly higher in the antibiotic treated batches. The ingesta required to produce one gram of larva, cocoon and shell were significantly lower in treated batches. Most of the feed conversion efficiency parameters related to digesta, such as efficiency of digested food into larvae, cocoon and shell were not significantly different between the control and the treated batches. The results indicated that higher assimilation and conversion of food was observed in silkworm treated with antibiotic.