The use of diffuse flat reflector to enhance the performance of flat-plate solar collector has been analyzed by both experiment on a water heating collector and its digital simulation. Fair agreement is obtained between experimental and simulated results for clear days. In this study the reflector covered with self-adhesive aluminum sheet （non-Cr-coated） has been placed below a collector optimally oriented at 52.3^o. An optimum reflector and collector orientation angle is found to be between 110^o and 120^o for fall and winter solar conditions. For typical fall and winter operating conditions, the addition of an optimally oriented diffuse reflector can result in an improvement in energy collection per unit area by more than 15 percent over that expected with an optimally tilted flat-plate collector. Finally, digital simulation has been used to estimate the monthly performance of a water heating collector with an optimal angle of the reflector.