This paper introduces a new flexible capacitive force sensor for cardiopulmonary resuscitation (CPR) to evaluate the quality of chest compression (CC). Compared with other feedback devices, this sensing element is flexible, which helps avoid the risk of tissue injury to the patient. We describe the relationship between the compressive force and the capacitance of the sensor element, and design a circuit to transduce the capacitance into a voltage. Finally, we tested the system during CPR training with a manikin. The experiment shows that the sensing element can help rescuers distinguish between effective and ineffective CC due to inadequate compression rate and/or depth.