Abstract The quality of silage affects the intake and utilization of ruminants. The use of crude glycerin, a by-product of biodiesel production, has potential as a silage additive to improve fermentation. This study aimed to evaluate the fermentation quality and chemical composition of Napier Pak Chong 1 grass ensiling at 60 days of cutting and chopping into 2-3 cm pieces with crude glycerin (CG) at 0, 10, 20 and 30%, which were assigned into a completely randomized design (CRD) with three replicates per treatment. The silages were packed in plastic bags under anaerobic conditions and fermented for 21 days to determine physical characteristics and chemical composition. All treatments produced very good quality silage with olive-yellow color and pleasant fermented odor. Increasing levels of crude glycerin significantly (p<0.01) improved fermentation characteristics through enhanced lactic acid production and reduced pH (4.64 to 4.26), while decreasing protein (12.16 to 4.00%), NDF (66.23 to 20.47%), and ADF (38.54 to 12.96%) but increasing NFE (44.28 to 79.44%). These results indicate that crude glycerin inclusion up to 30% significantly improves fermentation quality and reduces fiber content of Napier Pak Chong 1 silage, though protein supplementation would be necessary when feeding this silage to ruminants to achieve adequate nutritional balance.