In this paper, we present the fabrication and characterization of a silicon micromirror with carbon nanotubes (CNTs)-nickel (Ni) nanocomposite beams, and evaluate the mechanical stability of the micromirror in terms of resonant frequency. A novel electroplating method is developed for synthesis of the CNTs-Ni nanocomposite. CNTs are pretreated to have positive charges on their surface and added into a Ni electroplating solution to form a CNTs-Ni nanocomposite electroplating suspension. The weight fraction of the CNT in the electroplated nanocomposite is 9.1 wt%, and the ultramicroindentation hardness is 13 GPa. The mechanical strengthening improvement is found in the nanocomposite in comparison with a Ni film. The maximum variation of the resonant frequency of the micromirror during a long term stability test is about 0.4%, and its scanning angle is about 11°. It shows the potential ability of the CNTs-Ni nanocomposite with proper design for microstructure application.