Automatic detection of markers for fluoroscopic images is the key for fluoroscop y-based navigation system. Due to the small radius of markers and background no ises, the robust and the precision of traditional circle detect ion algorithms are not satisfied. In order to solve this problem, a geom etric-parameter based algorithm, which can extract markers automatically fr om X ray images, is proposed. The method can detect the circles of different ra dius in fluoro scopic images and non standard circles by adjusting the parameter. The e x perimental results show that the proposed algorithm holds high efficiency and robustness.