Background: Urtica cannabina, an unconventional forage, is widely distributed in northern China. It has high nutritional values that make it suitable for the ruminant’s feeding requirments as compared to Leymus chinensis. The current study was designed to evaluate varying ratios of Urtica cannabina and Leymus chinensis in the feeding diet and to see the effects on rumen fermentation and gas production in vitro. Methods: The study was designed into five treatments based on the different ratios of U. cannabina and L. chinensis: 0:100, 30:70, 50:50, 70:30 and 100:0 categorized into five groups from A-E. To detect the rumen fermentation parameters, the culture medium was collected at 1, 3, 6, 12 and 24 h. Result: Gas production of groups A and C was increased than other groups at 24 h (P<0.05), whereas the rate of gas production (c) was also increased in group A (P<0.05). The pH values at 1, 3, 6 and 24 h were increased in groups A and C with higher values in group C at 24 h (P<0.05). The ammonia concentration was increased in groups D and E at 3, 6, 12 and 24 h, with the lower values in group C at 24 h (P<0.05). The concentration of bacterial and protozoal proteins was also observed higher in groups A and C at 1 and 24 h, with highest value in group C at 24 h (P<0.05). In summary, as for Urtica cannabina to Leymus chinensis ratios are concerned, 50:50 is an optimal ratio for rumen fermentation in vitro, which increases the gas production and microbial protein synthesis. [ABSTRACT FROM AUTHOR]