Absract. A study with the purpose to increase microbial protein synthesis was carried out using in vitro experiment with 2x2 factorial. Completely Randomized Design were used in this study. The first factor was with or without substrate of Saccharomyces cerevisiae. The second factor was the addition of 2 levels of soybean oil, 0% and 3%. There were 4 treatments, each treatment was replicated 5 times so that there were 20 experimental units. The materials of this research were Napier grass, concentrates and substrate of Saccharomyces cerevisiae, consisted of a mixture of 30% rice bran, 20% tapioca by-product, 20% coconut meal, 20% corn, 9% pollard, 1% mineral, and soybean oil. The rumen fluid as a source of rumen inoculum was taken immediately after a cattle was slaughtered in the slaughterhouse. The variables measured were N-NH3 concentration and the product of rumen microbial protein synthesis. Data were analyzed using analysis of variance, followed by a test of Honestly Significant Differences (HSD). The conclusion was that the use of substrate of Saccharomyces cerevisiae increased rumen microbial protein synthesis as much as 9.42% and the use of 3% of soybean oil increased rumen microbial protein synthesis as much as 18.64%
