Modal bandwidth enhancement in multimode plastic optical fibers (POFs) is successfully achieved by a W-shaped refractive index profile, even compared to conventional high-bandwidth graded-index (Gl) POFs. We experimentally fabricate different types of W-shaped POFs by varying the index exponent g, numerical aperture, and depth of the index valley. Then, we verify that the group delay of high-order modes is strongly affected by the index valley, by which the bandwidth higher than that of conventional Gl POFs is achieved. In this paper, we design the optimum index profile of W-shaped POF for high-bandwidth performance. In addition to the bandwidth enhancement, we demonstrate low bending-loss in W-shaped POFs compared to Gl POFs.