<p>The utilization of tannin as an enteric-methane reducing additive has received wide attention in ruminant nutrition research but its astringency and the binding to digestible nutrients remain a major limitation. Encapsulation of tannin could mask the bitter taste while also delivering a controlled release of the bioactive into the rumen with ameliorated impact on feed intake and nutrient digestibility. This research aimed at evaluating the influence of encapsulating mimosa tannin with sunflower oil, and different inclusion levels into a substrate, on in vitro gas, methane, and organic matter digestibility, as well as rumen ammonia and volatile fatty acid concentration. The encapsulation process was done using solid-in-oil-in-water method. A dose comparison of the unencapsulated at 20 g/kg DM was compared with the encapsulated at 10–30 g/kg DM (EMTS¹⁰, EMTS²⁰ and EMTS³⁰)&#xa0;equivalent using two substrates a roughage (low digestibility) and a total mixed ration (high digestibility). The inclusion of UMT and EMTS decreased (<i>p</i> &lt; 0.05) gas, methane, methane/in vitro organic matter digestibility (IVOMD), gas/IVOMD, methane/total volatile fatty acid (TVFA) and gas/TVFA as well as gas yield from insoluble but slowly fermentable fraction, b and effective gas production (EGP). However, IVOMD was reduced (<i>p</i> &lt; 0.05) only by EMTS³⁰. While substrate type affected ammonia concentration, neither substrate nor tannin inclusion affected TVFA concentration (<i>p</i> &gt; 0.05). In conclusion, non-encapsulated and encapsulated mimosa tannin in sunflower oil decreased methane without any reduction in IVOMD, ammonia and total VFA at 20 g/kg DM inclusion and at 30 g/kg DM, encapsulated tannin decreased IVOMD further. The effectiveness of this encapsulation is recommended for other ruminant nutrition applications where targeted delivery of the active ingredient is desirable.</p>