The aim of current experiment was to determine the effects of grape pomace (GP) as carbohydrate source on the fermentation, aerobic stability, in vitro gas production, in situ rumen degradability characteristics and relative feed value (RFV) of alfalfa (Medicago sativa L.) silages in laboratory conditions. Grape pomace was applied at 40, 80, 120, 160 and 200 g/kg DM levels to alfalfa forage harvested at flowering stage and chopped to about 1.5-2.0 cm length. Supplementation of GP had a significant (P<0.05) effect on the chemical composition of alfalfa silage depending on the level of GP supplementation. The supplementation of GP significantly (P<0.05) decreased the crude protein (CP), ether extract (EE), neutral detergent fiber (NDF) and acid detergent fiber (ADF) whereas the ash, water soluble content (WSC) and total tannin (TT) increased with increasing level of GP. The supplementation of GP decreased the pH, acetic acid, butyric acid and ammonia-N (NH3-N) whereas the supplementation of GP increased the lactic acid and propionic acid content of alfalfa silage (P<0.05). The supplementation of GP significantly (P<0.05) increased the in vitro gas production, digestible organic matter (DOM), metabolizable energy (ME), in situ dry matter degradability (DMD), organic matter degradability (OMD) whereas crude protein degradability (CPD). On the other hand the supplementation of GP improved the aerobic stability of alfalfa silage decreasing the CO2 production. The supplementation of GP had also positive and significant (P<0.05). Effect on the relative feed value (RFV) and dry matter intake (DMI). As a conclusion the GP can be used as a silage additive in the level of 160 and 200 g/kg DM when alfalfa plant is ensiled.