The effect of substituting native grass with jengkol (Archidendron jiringa) by-product on fermentation characteristics, rumen microbial profile, methane production, and hydrogen balance using in vitro method was investigated. Seven treatments (different composition of native grass, jengkol peel, jengkol leaves, and concentrate) with five replications in a block randomized design were used. Substitution of native grass with jengkol peel powder up to 22.5% decreased rumen pH and protozoa, increased N-NH3, did not change feed digestibility (dry matter digestibility (DMD) and organic matter digestibility (OMD)), total and proportional volatile fatty acid (VFA) production, microbial protein synthesis, methane production, and hydrogen balance. The use of jengkol leaves powder up to 45% decreased rumen pH, increased N-NH3, feed digestibility (DMD and DMO) and microbial protein synthesis, but did not affect total and proportional VFA production, protozoa population, hydrogen balance, and methane production. It is concluded that native grass can be substituted with jengkol peel powder up to 22.5% and leaves powder up to 45%. Manuscript profile