Stretchable/flexible strain sensors are of great interests in the fields of human motion monitoring, soft robotics, and human machine interface. In this talk, we present stretchable strain sensors prepared by embedding the freestanding electrospun carbon nanofibers (CNFs) in an elastomer matrix (e.g. polyurethane). The piezoresistive response of the sensor under stretching was investigated; and the result showed that the sensor had high stretchability, high sensitivity to strain change, and good stability and reproducibility. The mechanism of the sensor was elucidated by morphological studies of the device at various strains. The effects of various device parameters on the device performance were explored to fabricate the strain sensors with sensitivity over a broad strain range. Furthermore, using these strain sensors as wearable devices for human motion monitoring was demonstrated.