In this paper, the optimal performance configurations of Solar Assisted Ground Source Heat Pump (SAGSHP) are proposed to provide the heat load requirements of a building in Tehran. Optimization has been done, on one side, based on the energy flow directions, introduced as control logic, and on the other side, using the optimal amount of energy from the solar and ground heat sources. Five scenarios are presented based on the control logic. In the following, the optimum mode in each scenario has been obtained according to the optimal percentage of energy available from considered sources in the studied area. In each scenario, the optimal mode is introduced based on the system's coefficient of performance (COP) which leads the optimal system sizing. For each scenario, the required heat is produced by the solar collector for the building, which varies from 20% to 60%, and the rest of the required heat demand is generated by the heat pump. The modeling results showed the fifth scenario as the best one for winter, where the working fluid from the ground heat exchanger and was directed into the solar collector, and its temperature increased through absorbing the heat of the sun. For summer, the suitable scenario where the solar collector and the thermal storage tank operate parallel to the heat pump is the fourth one. Finally and in general, the fifth scenario is considered as the optimal performance scenario, in which the COP in January has been 5.359.