Efforts were made to identify 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) and decrease in hydraulic retention times (HRTs) were utilized until the biogas volume became significantly decreased in SCFMR at mesophilic temperature (35 C). The optimum operating condition of the SCFMR fed with TS 15% dairy cow manure and saw dust mixture was found to be at HRTs of 30~ 35 days and its corresponding OLRs of 3.5~ 4.3 kgVS/m3-day. The optimum ranges of biogas and methane production rates were 1.36~ 1.47 volume of biogas per volume of reactor per day (v/vd) and 1.0~ 1.14 v/vd, respectively. This result was due to the high alkalinity concentration of SCFMR fed with the original substrate, dairy cow manure and saw dust mixture whose alkalinity was more than …