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Anaerobic digestion is basically a simple process carried out in a number of steps that can use almost any organic material as a substrate. To this end, attempt was made to investigate the effect of increase in organic loading rate on biogas productions using cow slurry as a single substrate at both mesophilic (37oC) and thermophilic (55oC) temperatures in a long time experiment with Continuously Stirred Tank Reactors(CSTR).. The OLR of 1.0goTS/l.d was started with and increased forthnightly by 0.5goTS/l.d until the OLR of 5.0goTS/l.d was reached after 140 days of running the experiment. It was observed that the biogas and methane yields decreased with increase in the organic loading rate. Both biogas yield and CH4 in the biogas decreased with the increase in OLR. For OLR in the range of 2.0 goTSl-1d-1 5.0 goTSl-1d-1, biogas and methane yields obtained were 0.435lN /g-0.300l/g and 0.251lN CH4/goS - 0.165lNCH4/goS, respectively at mesophilic temperature while biogas and methane yields obtained at thermophilic temperature were 0.420lN /g - 0.360lN/g and 0.239lN CH4/goS - 0.206lNCH4/goS respectively for the same range of OLR. Cow slurry was found to have an average methane content of 58% at both temperatures. Biogas yield y being a function of maximum biogas yield ym, reaction rate constant k and HRT were described on the basis of a mass balance in a CSTR and a first order kinetic. Using the value of ym obtained from the batch experiment with cow slurry, the values of k for both mesophilic and thermophilic temperatures were found to bed 0.176d-1 and 0.217d-1 respectively. The simple model equations were developed which can be used for dimensioning completely stirred tank reactors (CSTR) digesting organic wastes from animal waste slurries or biogas crops.