Lemongrass is the raw material for lemongrass oil production. Indonesia is one of the ten largest lemongrass oil producing countries in the world. Lemongrass oil industry will be generated large amount of lemongrass waste. The aim of this study was to determine the effect of elephant grass (Pennisetum purpureum) substitution with lemongrass waste on high forage feed on in vitro rumen fermentation. This study was arranged with randomized block design with 5 treatments (100R: 0% lemongrass waste, 75R: 25% lemongrass waste, 50R: 50% lemongrass waste, 25R: 75% lemongrass waste, and 0R: 100% lemongrass waste) and 4 different rumen collecting time as block. The results showed that the substitution of grass with lemongrass waste did not give a significant difference to dry matter content, but it could reduce protein content and increase crude fiber and crude fat significantly (P <0.05). The results of the in vitro analysis showed that lemongrass substitution did not affect pH, gas production rate, butyric acid, valeric acid, iso-butyric acid, and iso-valeric acid both in terms of quantity and proportion. The used of lemongrass by 25% (75R) was not affected on organic matter, potential gas production and methane production (24 hours) but decreased protein content and increased lag time significantly (p <0.05). The used of lemongrass by 50% was significantly reduce methane gas production (P <0.05). The increased of lemongrass waste concentration was affected partial VFA profile by decreasing in the amount of acetic acid. A significant decreased in the A/P ratio occurred in the used of lemongrass by 100%. Lemongrass waste could be used as elephant grass substituted up to 25% in high-forage feed without decreasing potential gas production, digestibility, and partial total VFA.