This study was conducted to test the effects of supplementing different oils on in vitro gas and CH4 production, ruminal fermentation, and digestibility. The study was carried out as a completely randomized design using rumen fluid obtained from three non-lactating Holstein Friesian dairy cows. The dietary treatments included: 1) high-concentrate diet without oil addition (Control), 2) linseed oil (LO), 3) sunflower oil (SO), 4) linseed oil and fish oil (LOFO), 5) sunflower oil and fish oil (SOFO), and 6) mixture of linseed oil with sunflower oil as well as fish oil (MIXO). The amount of added oils were at 3% DM. Cumulative gas production was recorded at 2, 4, 6, 8, 10, 12, 18, 24, 36, and 48 h incubation. In vitro digestibility was determined after 48 h incubation. Ruminal pH, NH3-N, VFA, and CH4 values were measured at 0, 2, 4, 6, and 24 h post incubation. Cumulative gas production at 48 h
incubation and protozoa population were lower (pSO). Inclusion of SOFO had lower total VFA concentration, lower acetate proportion, and higher propionate proportion than those of the control. Supplementation of LOFO and SOFO reduced (p(MCP) synthesis and in vitro digestibility of true DM, OM, and NDF. Based on this study, it is suggested that the inclusion of MIXO could maintain ruminal fermentation and digestibility, but could decrease gas and CH4 production.