The effect of ensiling duration on fermentation characteristics, chemical composition, gas production parameters and protein fractions of sweet sorghum silage was studied. Triplicate samples of sweet sorghum were ensiled for 0, 30, 60, 90, and 120 days in laboratory silos. Orthogonal contrasts were used to test linear, quadratic and cubic effects of ensiling time. The results of chemical analysis showed that,  ensiling  sweet sorghum increased dry matter (DM), soluble crude protein (SCP), ash and acid detergent lignin (ADL) concentrations compared to  fresh sweet sorghum (P<0.01), however, net energy for lactation (NEl) and organic matter digestibility (OMD)  decreased (P<0.01) compared to fresh material. The concentrations of ammonia nitrogen, lactic acid, acetic acid, ethanol and the amount of effluent increased significantly (P<0.01) with advancing ensiling period. The concentration of lactic acid was higher than other fermentation acids. DM and neutral detergent fibre (aNDF) concentrations decreased (P<0.01), while non-fibre carbohydrates increased linearly (P<0.01) with advancing ensiling process. The greatest amount of  water soluble carbohydrate (WSC) loss occurred within the first 30 day of ensiling (from 166 to 96.7 g/kg DM) and this trend continued quadratically (P<0.01) with  advancing ensiling time. Silages had lower (P<0.001) CP concentration compared with fresh forages (54.90 versus 61.80 g/kg DM) while time of ensiling did not affect (P<0.05) the CP concentration. Net energy for lactation (NEl) and organic matter digestibility (OMD) estimated from gas production technique were lower (P<0.01) for silages compared to fresh material and they decreased linearly (P<0.05) by advancing ensiling time. The results showed that, increasing ensiling time from 30 to 120 days decreased DM, WSC, aNDF and OMD concentrations and NEL content of sweet sorghum.