This study was carried out to evaluate the optimum operational condition of Semi-continuously Fed and Mixed Reactor (SCFMR) to treat the dairy cow manure and saw dust mixture. Step-wise increase in organic loading rates (OLRs) or decrease in hydraulic retention times (HRTs) were utilized until the biogas volume became significantly decreased at mesophilic temperature (). The optimum operating condition of the SCFMR fed with TS 13% dairy cow manure and saw dust mixture was found to be an HRTs of 25 days and its corresponding OLRs of . At this condition the biogas and methane production rates were 1.44 v/vd and 1.12 v/vd (volume of biogas per volume of reactor per day), respectively and the TVS removal efficiency of 37% was achieved. The successful operation with such a high OLR was due to the high reactor alkalinity concentration of 14,500~ 15,600 mg/L as  as a result of the characteristic of the original substrate, dairy cow manure and saw dust mixture whose alkalinity was more than 8,000 mg/L as . The parameters for the reactor stability, the ratios of volatile acids and alkalinity concentrations (V/A) and the ratio of propionic acid and acetic acid concentrations (P/A) appeared to be 0.11 and 0.43, respectively, that were greatly stable in operation. Free ammonia toxicity was not experienced due to the long term acclimation by the reactor TS content ranged 7.2~ 10.4% during the entire operational period.