The use of solar energy has increased in recent years due to the decliningfossil fuelresources and environmental concerns about global warming and air pollution.Due to lack of pollution, free and the availability, solar energy is considered one of the main promising alternative sources of energy to replace the fossil energy resources. Solar water heating systems are one of the main applications of solar energy whichin these systems, sunlight is transferred to water or working fluid.The most important part of a solar water heater system is the solar collector.There are several typesof solarcollectors, whichthe most common and popular kind of solar collectors is flat plate type.In this study, CFD analysis was used to investigate the effect of different parameters on thermal efficiency enhancement of flat plate solar collectors. Important parameters such as physical properties of the working fluid, pipe roughness, geometry of absorber plate, and thickness of the glass cover have been investigated. The results showed that by increasing the tube roughness, the collector efficiency increases. The thickness of the glass is not effective parameter on the thermal efficiency. The geometry and shape of absorber plate also was investigated. Flat and curved plates have been used and the numerical results showed that the efficiency of the flat absorber plate is higher than curved absorber. Key words: Flat plate solar collector, Solar hot water heater, Solar energy, Computational Fluid Dynamics (CFD)