A simple opto-mechanical system is introduced to monitor the fiber bending effects. Investigation of fiber rolling loss mechanisms in plastic optical fiber by intensity modulation of the transmitted light is reported. The transmitted light powers in the plastic fibers in bend-free and under bending deformation are measured and compared. By reducing the bending diameter (from 18 cm to 7.33 cm) the bending power loss is increased, accordingly. For the green LED for the force-free case the transmitted power is 7456.0 nW, while for bending diameter of 18 cm is 7349.8 nW, for bending diameter of 15 cm is 7291.4 nW, for bending diameter of 9.82 cm is 6687.6 nW and finally for the bending diameter of 7.33 cm is decreased to 5863.6 nW. For all the measurements the green LED shows the highest transmitted power while the red LED shows the lowest output power. Theoretically, the critical bending radius is computed as a function of the core index of refraction for a fixed value of the cladding index of refraction for different core radii. For the core radius of 430 μm the critical bending radius is about 12 cm, while for the core radius of 50 μm it is reduced to 1.8 cm. For a fixed refractive index of the cladding (1.43), the critical bending radius is decreased by increasing the core refractive index (1.44-1.52). The reported system provides a simple and accurate means for the bending loss investigation of fibers even for large-diameter bending cases.