I ndonesia is currently the world largest producer of oil palm products especially in the form of crude palm oil. This is possible since total oil palm plantation area in Indonesia is the largest among other countries, i.e. 8,150,000 ha, followed by Malaysia (4,620,000 ha), Thailand (720,000 ha), Nigeria (440,000 ha), Colombia (354,000 ha) and others (Garcia-Nunez et al., 2016). Oil palm plantation, apart from its main products, also produces significant amount of residual biomass such as oil palm trunk, oil palm frond, oil palm empty fruit bunch (OPEFB), kernel shell, mesocarp fiber and palm oil mill effluent (POME). Production of palm oil is approximately 10% from total biomass and the remaining is regarded as residual biomass (Ooi et al., 2017). Further, considering conversion factors from Stichnothe and Schuchardt (2010), processing of 100 kg of fresh fruit bunch (FFB) would result 20 kg of crude palm oil and 23 kg of OPEFB. On dry matter basis, two thirds of oil palm residue is originated from oil palm trunk and oil palm frond whereas one third is derived from FFB processing residues (Sulaiman et al., 2010). Such huge amounts of oil palm residues indicate their potency to be used as animal feeds particularly for ruminants since these residues (except POME) generally contain high proportion of fiber (cellulose, hemicellulose and lignin) but low research Article Abstract | This experiment aimed to enhance nutritional quality of oil palm empty fruit bunch (OPEFB) by combining urea treatment and high temperature and pressure (135 o C, 2.3 atm) using fiber cracking technology (FCT). The OPEFB was subjected to the following treatments: T1 (untreated OPEFB), T2 (OPEFB + FCT), T3 (OPEFB + 1% urea + FCT), T4 (OPEFB + 2% urea + FCT), T5 (OPEFB + 3% urea + FCT), T6 (OPEFB + 4% urea + FCT) and T7 (OPEFB + 5% urea + FCT), each in four replicates. Samples were determined for neutral detergent fiber (NDF), acid detergent fiber (ADF) and lignin contents, and were incubated in vitro with rumen fluid and buffer mixture. Results showed that treatment using FCT (T2) decreased NDF, ADF, cellulose and lignin contents of OPEFB. Combination between FCT and 1-5% urea (T3-T7) further decreased the fiber fractions, and addition 5% urea + FCT (T7) resulted in the lowest NDF, ADF, cellulose and lignin contents of OPEFB. Such fiber decrease of OPEFB due to FCT and urea was accompanied with significant increase of in vitro total gas production, gas production rate, total volatile fatty acid, ammonia, in vitro dry matter digestibility and in vitro organic matter digestibility as compared to control (P<0.05). However, methane emission was unaltered by FCT and/or urea treatments.