This study investigated high-solid codigestion of horse manure (HM) and grass in batch and semi-continuous systems. After 28 days, the cumulative methane yield of HM was 298.7 mL/g of volatile solids (VS). When the ratio of HM/grass was 80:20, on a percent VS basis, with the C/N ratio of 20.42 the highest methane yield of 382.3 mL/g of VS was achieved. The methane concentration of fresh HM was higher than that of codigestion and that of grass as a result of the high Volatile fatty acid concentration. The semi-continuous codigestion of HM and grass was carried out in a high-solid system with an organic loading rate (OLR) of 1.25, 1.88; and 2.50 g of VS L(-1)day(-1), which resulted in an average methane productivity of 152.9, 149.6, and 141.1 mL g(-1) of VS day(-1), respectively. The optimal OLR of semi-continuous codigestion was 1.88 g of VS L-1 day(-1), with the hydraulic retention time (HRT) of about 14 days. Thermogravimetric and differential thermal analysis (TG/DTA) revealed that More than 80% of the fractions of digestion residues were volatile matter.