Influences of the hydraulic retention time (HRT) and the organic loading rate onmesophilic and thermophilic anaerobic digestion of the organic fraction of municipal solid waste (OFMSW) were investigated by using a semi-continuous flow completely-mixed reactor. Theexperiments were conducted at a constant influent total solid (TS) of 10% by changing the HRT from 5days to 30 days under mesophilic (35°C) and thermophilic (55°C) conditions, respectively. Both themesophilic and the thermothilic digestion reactors were successfully operated even at a short HRT of 5days corresponding to a volumetric loading rate of 31.8 g-COD_Cr/L · day by adding mineral nutrient tothe feed. The results indicate that there was no significant difference between mesophilic andthernophilic digestion in terms of VS reduction, COD_Cr removal and methane production at the HRTs oflonger than 15 days, and the comparative merits of thermophilic digestion was found only at the HRTshorter than 15days. The water quality of the supernatant of the thermophilic digestion was worsethan that of the mesophilic digestion at the all conditions. Based on the comparison betweenmesophilic and thermophilic digestion at various HRTs, it is concluded that the HRT of thermophilicdigestion could be reduced to 7.5 days, but the mesophilic digestion should be operated at the HRT of 15days or longer for maintaining a high degradation efficiency of 75%.