A novel approach to enhancing optical absorption was reported by modulating the diameters of semiconducting nanowires, in which the diameter changes periodically in a sinusoidal manner along the long axis of the wire. 3D finite-difference time-domain (FDTD) simulations were used to calculate the optical properties of the diameter-modulated nanowires and compared the results with those for simple cylindrical nanowires and planar bulk silicon. The diameter-modulated silicon nanowires were modeled using a sinusoidal radial function with a period of 565 nm and modulated diameters of 495 and 380 nm at the convex and concave points, respectively, whereas the simple cylindrical silicon nanowire had a diameter of 410 nm. The optical absorption was calculated by measuring the power absorbed by the same volume of silicon over a propagation distance of 283 nm from the top surface, corresponding to the half-period of the diameter modulation. The results show that the optical absorption efficiency is highly enhanced with decreasing the mean diameter, and also increased with decreasing the modulation period, where the diameter difference is 115 nm from convex to concave points. On the other hand, the absorption effi ciency is not sensitive to the nanowire length of a few μm scale due to short propagation lengths less than 2 μm.