Off-design performance evaluation is one of the major topics of ORC-related investigations. However, there is a lack of adequate research on the influence of working fluid charge on system off-design performances. This paper presents the experimental results of a 3 kW(e) ORC prototype. Effect of three different working fluid charge on system key operating parameters have been investigated. The performance of system with different charge levels were measured and compared. Moreover, the pump cavitation and system performance fluctuation were discussed. The results indicated that overcharging working fluid will slightly improve the output power of the expander and results in sharp decline of vapor superheat by high shaft torque. The highest power output of the expander was obtained by 40% pump stroke, and the calculated and measured values were 2.79 kW(e), and 2.28 kW(e), respectively. Furthermore, the isentropic efficiency of the pump can be also improved by overcharging working fluid. However, such improvement was more obvious with smaller pump stroke. Meanwhile, undercharging working fluid will lead to insufficient liquid level in condenser, and pump cavitation has been detected. Therefore, plenty amount of working fluid charge is necessary to maintain a certain liquid level in the condenser and to avoid pump cavitation. (C) 2019 Elsevier Ltd. Alt rights reserved.