Enterococcus faecium had been widely used as direct fed microbial due to their capability to produce antibacterial compounds. Thus, this study was conducted to determine the beneficial effects of E. faecium on in vitro rumen fermentation, methane concentration, microbial diversity and population. Ruminal samples were collected from ruminally cannulated Holstein Friesian cattle and 40:60 rice straw to concentrate ratio were used as substrate at 1g dry matter (DM) per 100mL buffered rumen fluid. Fresh culture of E. faecium (7.5 × 10⁸ cfu/ml) at different inclusion rates were investigated using in vitro rumen fermentation. The following treatments were non addition, 0.1%, 0.5% and 1.0% of E. faecium and, hereafter referred to as control, treatments 1, 2, and 3, respectively. Increased in total gas production and ammonia nitrogen concentration over time were found in control and all treatments while the opposite was observed in pH. Addition of E. faecium had significant effect on total gas production after 48h of incubation having the highest (P < 0.05) in T2 followed by T4, T3 and control with 93.33, 91.75, 89.50 and 82.20 ml, respectively. pH was not affected by addition of E. faecium and lactate was only detected at 0h and was not detected after 12, 24 and 48h of incubation. Propionate was found highest (P < 0.05) in T1 after 12h of incubation with 14.15 mM/L. Higher (P < 0.05) concentrations in treated than control were found in acetate after 12h, propionate after 48h, butyrate after 12, 24 and 48h and total volatile fatty acids (TVFA) after 12 and 48h of incubations. Pyrosequencing, methane concentration, and methanogen quantity are still on going. So far, addition of E. faecium did not affect the pH but increased the propionate and TVFA concentrations.