Periodic Si nanopillar arrays (NPAs) were fabricated by the colloidal lithography combined with catalytic etching. By varying the size of colloidal crystals using oxygen plasma etching, Si NPAs with desirable diameter and fill factor could be obtained. The Fresnel reflection can be eliminated effectively over broadband regions by NPAs; i.e., the wavelength-averaged specular reflectance is decreased to 0.70% at wavelengths of 200-1900 nm. The reflectance is reduced greatly for the incident angles up to 70 degrees for both s- and p-polarized light. These excellent antireflection performances are attributed to light trapping effect and very low effective refractive indices, which can be Modified by the fill factor of Si in the NPA layers.