The organic Rankine cycle (ORC) is one of the key technologies to fulfill the efficient utilization of low grade heat sources. Working fluid circulating pump is the main power consumption component of ORC system. However, the overall pump efficiency as low as 7%~20% was reported in the experimental study. To solve this problem, an experimental system for heat recovery from low grade heat source was constructed, R123 was selected as working fluid, a scroll expander was used to produce work, and a tri-plunger pump was selected as working fluid circulating pump. The conductive oil was heated by electric heater to simulate low grade heat source. The maximum expander shaft power and thermal efficiency were 2.16 kW and 5.35% at the heat source temperature of 140℃, respectively. The range of overall pump efficiency was about 22%~30%, which was 1.5~4.3 times of that reported in the references. The isentropic efficiency and mechanical efficiency of the pump were 60%~69% and 37%~45%, respectively. Analysis results showed that the reason for low mechanical efficiency of the pump was due to the fact that the pump operating parameters were deviated far from the rated parameters. Additionally, cavitation could weaken pump isentropic efficiency and reduce the working fluid flow rate at the same time. Critical sub-cooling temperature as high as 21℃ was obtained at the pump inlet to prevent the cavitation. The study results identified that tri-plunger pump can be recommended in the ORC system.