Organic Rankine Cycle (ORC) power systems convert low-temperature thermal energqy into electrical energy, which is a potential technology for waste heat recovery and renewable energy utilization. To improve the performance of small-scale ORC power systems, the relationship between system performance and load resistance in ORC should be further investigated. An ORC test-rig with a scroll expander directly connected to a generator was built. With a variable-temperature heat source, working fluid R141b and R245fa, and electrical load, the effects of the load resistance (50 Ω to 200 Ω) and the heat source temperature (75 °C to 95 °C) on the performance of the ORC system were investigated. The results show that the optimum value of load resistance exists for the ORC system to maximize the output power or thermal efficiency, and this value varies with the heat source temperature and working fluid. For R245fa, the optimal load resistance to achieve the maximum shaft output power changes from 100 Ω to 200 Ω when the heat source temperature reduces from 95 ℃ to 75 ℃. For R141b, the optimal load resistance to achieve the maximum shaft output power is fixed around 200 Ω. The analysis shows that the changes of load resistance affect the operating conditions of the scroll expander, which in turn affects the flow rate and heat transfer of the system. Therefore, the change of load resistances has a combined effect on system performance. This study highlights the importance of matching the load resistance, heat source temperature, and working fluid, and provides experimental data for optimizing ORC power systems.