This paper presents a novel tool path planning approach for polygonal mirror scanning based stereolithography (STL) process. Compared with traditional laser scanning and mask projection based STL process, the polygonal mirror scanning based process can build part with high surface quality and precision without losing the fabrication efficiency. As an emerging additive manufacturing (AM) process, no efficient tool path planning algorithm is available in current system. This paper presents a direct tool path planning algorithm without converting the three-dimensional model into two-dimensional contours. Different test cases are used to verify its efficiency and effectiveness. Compared with the commercial software, the proposed algorithm is several times faster. Physical parts are also built using the tool path generated by the proposed algorithm.