Detecting and monitoring of crack in viscoelastic solids, as a result of large and time-dependent deformations, are of great importance but have not been carried out sufficiently well. In this paper, polymer optical fibers (POFs) are employed to detect and monitor cracks in viscoelastic solids subjected to tensile loading. The sensor system is developed based on the variation of light intensity within the POFs when the POF sensors are loaded. The sensors show good stability, cost-effective, and large enough strain range (60%). The results demonstrate that in addition to monitoring the strain state of viscoelastic solids, the POFs strain sensors can detect initial cracks and monitor cracks propagation up to ultimate failure.