Biogas is a good source of renewable energy.This research work has been done for the design, fabrication, and investigationof a digester with solar thermal flat plate collector to heat slurry inside thedigester in order to maintain the temperature in appropriate range (37±2). Forproduction of continuous biogas in winter, a minimum temperature is required forthe slurry 7kg COD/m3 organic waste. The Hydraulic Retention timefor this research experiment was 12 days. Flat plat thermal collector is usedin this research to heat water and circulate it inside the digester. The hotwater is circulated by copper coils during the winter season. By adopting thismethod the temperature of the slurry enhanced to required minimum temperatureand the production of biogas was continued through the whole year in the colderarea. The data collected from the experiment showed that using of flat platthermal solar collector is the appropriate method for heating slurry to producebiogas through the whole year in the colder area in winter season.