This study aimed to evaluate the antiprotozoal activity of potato peels and linseed oil. The material used were complete feed, potato peel and linseed which was experimented in vitro with four treatments and one control, T0; T1 (complete feed + linseed oil 0.15%); T2 (complete feed + 0.15% potato peel); T3 (complete feed + linseed oil 0.075% + potato peel 0.075%); T4 (complete feed). The results obtained showed a significantly different (P<0.01) on protozoa defaunation, total gas and degradable Dry Matter (DM), and Organic Matter (OM). The addition of potato peel and linseed had the same defaunation effect but tended to decrease, while the total gas production and DM and OM degradability differed. The addition of 0.075% potato peel and 0.075% linseed oil in the complete feed (T3) resulted in the most efficient treatment for gas production, DM, and OM degradability value. In conclusion, the use of plant bioactive compounds as biomarkers in the manipulation of rumen fermentation needs to consider several things, especially the condition of the rumen microflora, instead of focusing on the final results of the manipulation.