This paper presents a steady state simulation model to analyze a direct expansion ground source heat pump that uses carbon dioxide as refrigerant in a transcritical cycle. The analysis considers pressure drop characteristic in heat exchangers of the system. The present numerical model has been developed to examine the system in different operating conditions in two discrete cases. These cases include constant evaporator loop length and specified heating capacity. Then model evaluate the system based on COP and heating capacity in the first case and COP and evaporator loop length in the second case. To evaluate the performance of the system, a parametric study is performed to investigate the effect of different parameters such as difference between soil temperature and evaporator outlet temperature, compressor speed, water inlet temperature, water mass flow rate, area ratio heat exchanger and soil temperature. In both studies, It can be concluded that in a specific temperature difference of the difference between soil temperature and the outlet temperature of the evaporator, there is a maximum value for COP. Also, increasing the compressor speed and the temperature of inlet water to the gas cooler is accompanied with the diminution of COP. Increasing the mass flow rate of inlet water to the gas cooler leads to surge in COP. The results of this study include heat capacity, coefficient of performance and ground heat exchanger loop length can be used for design and optimization of a direct expansion geothermal heat pump.