Ordinary silica fibers are easy to break in real applications due to their characteristics of brittleness. Plastic optical fibers were proposed to replace silica fibers in crack monitoring. Moreover,considering the uncertainty of crack propagation direction in composite materials,the influence of the angles between plastic optical fibers and cracks was studied. A set of plastic optical fiber sensing device was designed and the relationship between optical loss and crack width under different fiber/crack angles was measured through the device. The results show that the optical loss in plastic optical fibers increased gradually with crack propagation. Therefore,applying plastic optical fibers in crack monitoring is feasible and the sensitivity of plastic optical fiber crack sensors decreases with the increasing of fiber/crack angles. Moreover,from the experimental results,the maximum optical loss of 0.5mm plastic optical fibers is 78.76 μW,while the maximum optical loss of 0.25 mm plastic optical fibers is only 24.28 μW. Therefore,the fibers with bigger diameter have more bend loss when they are bent.