Abstract: The objectives of the present study were to evaluate the potential of fermented palm press fibre (FPPF) as supplement feed to increase in vitro rice straw digestibility. Complete randomized design experiment was used to determine digestibility (in vitro) of rice straw supplemented with palm pressed fiber. Three treatments were composed as control (P0), rice straw supplemented with 5% FPPF (P1) and rice straw supplemented with 10% FPPF (P2) with 6 replication for each treatment. Observed parameters were digestibility of dry matter (%), organic matter (%), crude protein (%), crude fiber (%) ammonia-N (mM) and Total VFA (mM) concentration. Rumen bacteria population post 24 h incubation were also determined. The result showed that digestibility of dry matter and crude protein were not altered by treatment (p>0.05) while organic matter and crude fiber were significantly (p<0.05) affected by PPFP supplementation. Rumen bacteria population post incubation were not significantly (p>0.05) altered by PPFP supplementation but the number tends to increase.