A textile-based strain sensor for measuring the length of a McKibben pneumatic actuator has been developed. McKibben actuators are flexible, lightweight, and widely used in all those applications where compliance and safety are required, e.g. soft robotics and power assisting device. The actuator length needs to be measured to control the device accurately. However, properties such as flexibility and lightness might be lost if rigid sensors such as potentiometers or linear encoders are directly attached to the actuators. For this reason, flexible and stretchable sensors are necessary. In this study, a flexible sensor using conductive textile is proposed to actively measure the length of manufactured McKibben actuators. Firstly, the electro-mechanical characteristics of the proposed sensor were obtained and a model to compensate its nonlinearities was evaluated. Secondly, an estimation of the accuracy was performed during dynamic actuator contractions. The results showed that, using this sensor, a direct measurement of the actuator axial displacement can be obtained within 20% error, without affecting its performances in terms of contraction.