We investigated optical properties of planar Si wafers and Si microwire (MW) arrays with and without ZnO thin films using the finite-difference time-domain (FDTD) method. Reflectance of the MW array (diameter: 4 μm and period: 12 μm) was smaller than that of the planar wafer in the wavelength range from 400 to 1100 nm, which could be originated from antireflection effects due to low optical density and guided-mode-assisted field enhancement. The reflectance of ZnO (thickness: 50 and 80 nm)-coated MW array was drastically reduced compared with the bare array but somewhat larger than that of the coated planar wafer. This could be attributed to less-confined guided modes in the wires, which was supported by the field distribution simulation results. Our results provide some insights into possible roles of transparent conducting layers on MW arrays for photovoltaic applications.