This study aimed to evaluate effects of feeding oils and condensed tannin on in vitro true digestibility (IVTD), ruminal fermentation and methane (CH₄) emission in dairy cattle. Four donor heifer Holstein Friesian cows were fed a diet including concentrate and elephant grass (40:60, DM basis) for a 7-day duration. The cows were then collected rumen fluid before morning feeding to conduct in vitro incubation. The in vitro study was a completely randomized design with 7 treatments and 4 replicates per each treatment. A basal diet (BD) including concentrate and elephant grass at 40:60 was used as a control; meanwhile, other 6 diets were substitution of BD by 2.5% oil (either only soybean oil [S] or a blend of soybean oil and tuna fish oil [SF]) combined without or with grape seed extract (a great source of condensed tannin [T]) at 0, 0.4 and 0.8% DM, corresponding to Ctrl, ST0, SFT0, ST0.4, ST0.8, SFT0.4 and SFT0.8, respectively. Grape seed extract was a commercial product contained 95% condensed tannin in the form of proanthocyanidins (PA). Results shown that SFT0.4 had higher NH₃-N, VFA and IVTD compared to those in SFT0 at 24h after incubation (P4 production by 23.97, 26.70 and 22.00% after 48h incubation (P4 production, but it caused negative effects on ruminal NH₃-N, VFA and IVTD at 24h incubation; meanwhile, a reverse trend was observed in SFT0.4. Conclusion, SFT0.4 is an ideal diet to reduce CH₄ emission, improve ruminal fermentation and digestibility in dairy cows.