Micro-mirror is the key structure of MEMS optical switch, Digital Mirror Display (DMD) and variable optical attenuators. Surface roughness plays a crucial rule in reflectivity, insertion loss, and all kinds of surface forces. For describing the topography of surfaces, there are several roughness parameters. Some parameters depend on sample size and instrument resolutions while fractal theory, which is characterized by continuity, non-differentiability and self-affinity, shows its complementarities. In this paper, the topographies of various micro-mirror surfaces are measured by Scanning Tunneling Microscope (STM). Then, these rough surfaces are charactering by a fractal theory, and a three-dimensional Weierstrass-Mandelbrot function is applied to simulate the topography of the rough surfaces. The result shows that the Weierstrass-Mandelbrot function is efficient in creating models and analysis the topography of MEMS surface. It will be very useful in the future researches.