Dry etching of silicon is an essential process step for the fabrication of Microelectromechancal system (MEMS) The AZ7220 positive photo-resist was used as the etching mask and silicon micro-trenches were fabricated with a multiplexed inductively coupled plasma (ICP) etcher.The influence of resist pattern profile, and etch condition on sidewall roughness were investigated detail. The results show that the sidewall roughness of micro-trench depends on profiles of photo-resist pattern, the initial interface between the resist bottom surface and silicon surface heavily. The relationship between roughness and process optimization parameters are presented in the paper. The roughness of the sidewall has been decreased to a 20-50nm with this experiment.