This study investigated the co-digestion performance of Pennisetum hybrid and cow manure via batch mode tests to optimize the feedstock ratio. On the basis of the batch test results, the long-term operational performance of the co-digestion system at different ratios was assessed at mesophilic temperature and organic loading rates (OLRs) of 2.0-5.5 g of volatile solids (VS) L-1 day(-1) via semi-continuous mode. Batch tests showed that the maximum specific methane yield of 170.35 +/- 11.61 mL/g of VS was obtained at a Pennisetum hybrid/cow manure ratio of 100:0, with the specific methane yield decreasing with an increased proportion of cow manure. In the semi-continuous experiments, the mono-digestion of Pennisetum hybrid failed at the OLR of 2.0 g of VS L-1 day(-1). This failure was accompanied by the accumulation of volatile fatty acids (VFAs). The stable operational OLR increased by 1.0-1.5 times with the addition of cow manure. The volumetric biogas yield increased with the augmentation of the OLR from 2.0 to 4.5 g of VS L-1 day(-1), while the specific biogas and methane yields both decreased. A comparison of co-digestion systems with 75 and 50% Pennisetum hybrid revealed that higher proportions of grass contributed to enhancing both the conversion efficiency and specific methane yield. The accumulation of total ammonia nitrogen and VFAs was found to trigger co-digestion instability at higher OLRs.