In this study, a medium scale biogas digester has been operated to treat the cattle manure of 50 animals producing about 1000 kg of manure per day. The volume of digester is 200 m³ and was designed to produce biogas for a 50 kWh/day electric generator and 28 day the hydraulic retention time (HRT) with daily feeding rate 6 m³ of substrate. From the evaluation results, the daily feeding rate is 5 m³ and HRT of 40 day. The biogas yield reached 11.51 m³/day and the CH₄ and CO₂ concentration ranged from 37 % to 61 % and from 13.3 % to 37.8 %, respectively. On the other hand, a reduction in the organic contents of substrates is explored because of the anaerobic digestion process. These contents are volatile fatty acids (VFAS), carbon to nitrogen ratio (C/N), volatile solids (VS), and chemical oxygen demand (COD), where the maximum reduction percentages of VFAS, C/N ratio, VS, and COD are 78.2 %, 76.7 %, 76.5 %, and 87.0 %, respectively. Furthermore, the concentration of total nitrogen (TN), total phosphorus (TP), total potassium (TK), NO₃-N, and NH₄+N achieved an increase in the digestate compared with their corresponding values in substrates, especially TP and TK. From the evaluation results, we can put and illustrate some important issues that may increase the biogas digester efficiency; (1) the biogas purification unit should be fixed and operated to reduce the CO₂, H₂S and O₂ contents, (2) The substrate pH, and total solid should be more controllable, (3) The digestates should be treated before use for further benefits and more stable with environment, and (4) a periodically maintenance plan should be followed for improving biogas production and increase the life time of the digester components.