Abstract: Digital reconstructed radiograph (DRR) is an important technique widely used in radiotherapy including diagnosis, pre-operative planning and intra-operative assistance. In this paper, we present a novel two-step hardware accelerated DRR algorithm. First, three stages involved in direct volume rendering algorithm, namely, geometric transformation, traversing and illumination, are fully accomplished in graphics accelerator, yielding an X-ray attenuation image. Subsequently, the resultant image is used for simulating X-ray exposure process by exploring the newest features such as render-to-texture technique. Preliminary experimental results show that our algorithm can achieve high-quality DRR images at an interactive rendering speed and thus is suitable for radiotherapy diagnosing and treatment applications.