In this paper, a 1 kW ORC experimental system is built. Using R123 as the working fluid, transient responses of Basic ORC (BORC) and ORC with a regenerator (RORC) are both tested under critical conditions. A total of four experiments are carried out, including: (1) Case 1: the working fluid pump is suddenly shut down; (2) Case 2: the working fluid is overfilled or underfilled; (3) Case 3: the torque of the expander is suddenly loss. (4) Case 4: the cooling water pump is suddenly shut down. All the major quantities such as the output power and torque of the expander, temperatures and pressures at the inlet and outlet of the expander, temperatures at the inlet and outlet of the condenser are measured. The transient responses of the two systems under the controlled critical conditions are tested and compared, some physical explanations are provided. It is found that RORC is more stable than BORC because of the regenerator. Regenerator should act as a "pre-heater" or "pre-cooler" under the critical conditions thus improving the stability of RORC. When the working fluid in the system is underfilled or leaked, the system performance is extremely unstable. Otherwise, when the working fluid is overfilled, the trend of the curves are similar to the optimal working condition but with weaker performances. We also find that if the working fluid pump is shut down when working fluid is overfilled, the rotation speed and shaft power output of the expander will increase significantly, the unique phenomenon can be used to estimate whether the working fluid in the system is overfilled. (C) 2018 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.