Abstract Feeding herbs to dairy cattle may effect rumen fermentation and digestibility due to the herbs’ physiological or pharmacological functions. Thus, in the present study, peppermint was tested for its effects upon digestibility and rumen fermentation. Digestion trials and an investigation of rumen fluid were conducted by four rumen-cannulated Holstein steers having or not having been fed peppermint. When peppermint was fed to the steers, the digestibility of nutrients tended to be higher than that of the control. Ammonia-nitrogen concentrations were higher in the control steers than in the peppermint-fed steers ( P<0.01). Numbers of Entodinum, Isotrica, and Diplodium were decreased significantly by the ingestion of peppermint ( P<0.05). The total numbers of protozoa were significantly decreased by peppermint ingestion ( P<0.05). Based on these results, it can be concluded that peppermint has a great potential as a natural manipulator of rumen fermentation by depressing ammonia-nitrogen concentration or numbers of protozoa.