This paper presents the results of theoretical and experimental investigation to determine the optimal flow rate of collector loop to improve the performance of a solar hot water system under real weather conditions at Ho Chi Minh City, Vietnam. The optimal flow rate of flat-plate collector loop is researched based on the relationship between the useful heat gain of the flat plate collectors and the electricity consumption of the collector pump. The result of the simulation and experiment shows that the optimal flow rate was determined at the value of 0.132 (kg/s). Besides, the effects of various parameters such as initial water temperature of storage tank, flat plate collector area and volume of storage tank are investigated. Furthermore, the validation of simulation and experiment results of the progress of the water temperature in the storage tank is also evaluated.