This experiment was conducted to investigate the effects of different dosages and types of medium‐chain fatty acids (MCFAs) on rumen fermentation in vitro under low‐ and high‐concentrate diets. For this purpose, two in vitro experiments (Exp.) were conducted. In Exp. 1, the concentrate–roughage ratio of the fermentation substrate [total mixed rations (TMR), dry matter (DM) basis] was 30:70 (low‐concentrate diet), while in Exp. 2, it was 70:30 (high‐concentrate diet). Three types of MCFAs with octanoic acid (C8), capric acid (C10), and lauric acid (C12) were added accounting for 1.5%, 6%, 9%, and 15% of the in vitro fermentation substrate weight (200 mg or 1 g, DM basis) based on control group, respectively. The results showed that the addition of MCFAs all could significantly reduce methane (CH4) production and the number of rumen protozoa, methanogens, and methanobrevibacter under the two diets with the dosages increased (p < 0.05). In addition, MCFAs had a certain degree of improvement on rumen fermentation and influenced in vitro digestibility under low‐ and high‐concentrate diets, and their effects were related to the dosages and types of MCFAs. This study provided a theoretical basis for the selection of types and dosages of MCFAs in ruminants production.

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