This study investigates the performance of anaerobic co-digestion of a mixture of food waste (FW) and domestic wastewater (DWW) (at 0.185 ratio, v/v) without and with addition of magnesium (in forms of MgSO4 and MgCl2) at different concentrations of Mg2+ (50, 150, and 250 mg/L). The upflow anaerobic sludge blanket (UASB) reactors were operated under the mesophilic condition (35℃) at 20 days of hydraulic retention time (HRT) with the intermittent feeding (48 hours feed and 48 hours no feed). The best performance of co-digestion in terms of organic matter removal and methane production was observed at 150 mg Mg2+/L supplementation. The chemical oxygen demand (COD) removal efficiency and methane content reached 81±2.2% and 60.8±1.2%, respectively, which were 10.2% and 9.6% higher than the control without Mg2+ addition, as well as lower concentration of ammonia-nitrogen (672.1±51.3 mg/L) accumulated compared to the control (811±44.4 mg/L). Supplementation of Mg2+ was observed to have effect on removing ammonia-nitrogen which would inhibit anaerobic digestion (AD) to some extent.