1. A tracking system for region-of-interest (ROI), comprising: a feature point detecting and tracking module that performs feature-point detection locally on an ROI of an image frame at an initial time and tracks a detected at least a feature point; a re-detection module that sets a re-detection condition for performing a feature point re-detection in said ROI at a current time to obtain a tracking result within a stability range; wherein said re-detection condition for performing a feature point re-detection is when a number of said detected at least a feature point is less than a pre-set percentage of a number of initial feature points; a linear transformation module that finds out a transform relationship between two ROIs of two consecutive image frames, by using a plurality of corresponding tracked feature points; an estimation and update module that predicts and corrects a moving location for said ROI at a current time; and an outlier rejection module that removes at least an outlier outside of said ROI, based on a result corrected by said estimation and update module.