Abstract: Nine, two-years old of crossbred native cattle were used to examine the effects of supplementation levels of palm oil in yeast culture fermented cassava pulp on rumen fermentation and average daily gain. The cows were randomly allocated in a complete randomized design and three replicates (animals) per treatment. The first group (control) was fed on a yeast fermented cassava pulp without palm oil (YFCP0), The second group was fed yeast fermented cassava pulp + palm oil at 1% (YFCP1)and third groups was fed yeast fermented cassava pulp + palm oil at 2% (YFCP2), respectively. The cows were offered the treatment diets at 2%BW and rice straw was fed ad libitum. The results have revealed that supplementation of dietary treatment on feed intake, ruminal pH, ammonia-nitrogen and blood urea nitrogen concentration were non-significantly different, while Average Daily Gain (ADG) and cost production were significantly different and had highest in cattle receiving YFCP2 than those fed YFCP1, YFCP0 diets (633.1, 614.5 and 511.1 g/day of ADG and 0.92, 0.81, 0.73 US$/kgBW of cost production, respectively). The populations of bacteria and fungal zoospores were significantly different as affected by levels of palm oil supplementation. Especially, supplementation of YFCP2 in cattle had highest increase populations of bacteria and fungi zoospore than those fed YFCP1 and YFCP0 but decreased protozoal populations. Therefore, supplementation levels of palm oil at 2% in Yeast Culture Fermented Cassava Pulp (YFCP1) as supplement diets with rice straw as roughage source could highest improved ruminal fermentation efficiency, average daily gain including increase populations of bacteria and fungi zoospores, but decreased protozoal populations in rumen of crossbred native cattle.