Abstract: An object tracking algorithm based on planar projective invariants is proposed,in which the values of invariants for object modeling and tracking are calculated with lines extracted from images.In order to attain the line edges in images,all edges are thinned by improved sequential thinning algorithm to be one pixel wide at first.Then a fast computing method is presented to estimate the curvature of edge points and the points at which curvature is less than some threshold are collected to fit lines.Lines are selected by neighborhood-rule or window-rule to compute projective invariants.In the image processing experiments, the straight lines are extracted with the image preprocessing algorithm proposed in this paper,and the stability and viewpoint-invariance of the invariants calculated by those lines are assessed.The tracking experiments verify the robustness and practicability of the algorithm.