This paper presents a new sidewall smoothing process for Si micro-optical components using focused ion beam (FIB) milling. First, the deep reactive ion etching (DRIE) Bosch process is employed to form a microstructure on a Si substrate. However, scalloping which is induced on the sidewall by the repetition of etching and passivation steps is a major source of light scattering, reducing surface reflectivity in optical applications. In this research, FIB is used to smooth the rough sidewall surface and obtain polished mirror surface. After the DRIE Bosch process, the values of sidewall roughness as measured by atomic force microscopy were 153.0 ± 13.5 nm with an rms value of 28.1 ± 6.4 nm. The FIB smoothing process tremendously improved the surface roughness of etched sidewalls resulting in a maximum peak-to-valley roughness and an rms roughness of 5.7 ± 1.8 nm and 1.6 ± 0.7 nm, respectively. In this paper, the DRIE Bosch process and the FIB smoothing process are described in detail and applications using this technique are discussed.