Fat or oil supplementation in the ration has a negative effect on rumen microbes and biohydrogenation of fat. Protection of fat is expected to reduce these negative effects in rumen. Meanwhile, excess degradation of high-protein feed in the rumen can reduce the efficiency of post-rumen nutrient utilization. Protection of protein is expected to increase protein absorption in the intestine. This study aimed to determine the rumen fermentation parameters of a mixture of soybean meal, saponified crude palm oil (CPO) and Agromix® mineral premix which was protected with formaldehyde. The crude palm oil used is protected by the saponification method and mixed with soybean meal and Agromix® mineral premix. The mixture of these three materials was then protected with formaldehyde (formalin 0.8%). An independent sample T-test was used to compare the 2 treatments (T1/control=commercial ration and T2=commercial ration with 4.4% protected feed substitution). All treatments were incubated for 48 h according to Tilley and Terry's in vitro technique. The results showed that the treatment did not affect the rumen pH. Compared to T1, T2 decreased (p<0.05) N-NH3 and CMC-ase enzyme activity, but did not affect the concentration of rumen microbial protein (p>0.05). In addition, T2 treatment decreased (p<0.05) VFA concentration but did not affect the A:P ratio. It was concluded that the substitution of protected feed could increase the efficiency of ruminant feed containing high protein as evidenced by a decrease in the value of NH3 but did not affect the pH value or rumen microbial protein. [ABSTRACT FROM AUTHOR]