Direct-expansion solar-assisted heat pumps (DX-SAHP) have been used widely to heat the consuming water of buildings and industrial facilities, domestic and industrial space heating and also, air conditioning. These systems transfer energy from lower temperature source to a higher temperature source. In DX-SAHP systems, In order to optimize the heat transfer of solar radiation to the refrigerant, the flat plate solar collector is used as the evaporator. In this paper, the thermal performance of a DX-SAHP has been studied using numerical simulation for heating the water of a house in Kermanshah. The system mainly employs a bare flat-plate solar collector with a surface area of 4 m2, a hot water tank with the volume of 150 L, a rotary-type hermetic compressor, a thermostatic expansion valve and R-134a is also used as working fluid in the system. The results show that the hours of system operation, during different months in the climate of Kermanshah, vary between 37 to 130 hours and the monthly average COP and the solar collector efficiency vary between 3.96 to 6.71 and 68 to 99 percent respectively. The effect of various parameters, including solar radiation, ambient temperature, collector area, compressor speed, number of collector cover and wind speed have been analyzed on the thermal performance of the system.