This study was planned to determine the chemical composition, protein fractionation and fermentation characteristics of sweet sorghum and sweet sorghum bagasse silages treated with urea and molasses. Treatments were maize silage (MS), sweet sorghum silage (SS), sweet sorghum bagasse silage (BS) and/or urea and molasses supplemented (10 and 50 g/kg dry matter (DM) basis, respectively) SS and BS. Triplicate silage samples were prepared for each treatment in laboratory silos. Fresh sweet sorghum and its bagasse had greater (P<0.01) DM and NFC, but lower (P<0.01) CP and a NDF concentrations compared with maize. After 90 d of ensiling, similar results were found in produced silages. The SS and BS had lower protein “A” fraction but greater protein “C” fraction (P<0.01) compared with MS. Treating of SS and BS with urea or molasses resulted in an increased (P<0.05) in pH. Orthogonal comparisons showed adding urea increased CP, lactate, ammonia nitrogen (NH₃-N) concentrations, and pH value but decreased NFC (P<0.01). Adding molasses to SS and BS silages increased DM, WSC, NFC, pH (P<0.01) and in vitro digestibility of DM (P<0.05) while caused a decrease in a NDF, ADF, acetate (P<0.01) and ethanol (P<0.05). Adding of urea or urea plus molasses to SS and BB resulted an increase in protein “A” fraction (P<0.01) but a decrease in protein “C” fraction (P<0.01). Therefore, simultaneous application of urea and molasses improved the nutritional quality, DM digestibility, protein fractionations and fermentation characteristics of SS and BS and offered a potential alternative silage crop compared with maize in the arid areas.