Calibration and registration are the first steps for augmented reality and mixed reality applications. In the medical field, the calibration between an RGB-D camera and a mobile C-arm fluoroscope is a new topic which introduces challenges. In this paper, we propose a precise 3D/2D calibration method to achieve a video augmented fluoroscope. With the design of a suitable calibration phantom for ... [Show full abstract] RGB-D/C-arm calibration, we calculate the projection matrix from the depth camera coordinates to the X-ray image. Through a comparison experiment by combining different steps leading to the calibration, we evaluate the effect of every step of our calibration process. Results demonstrated that we obtain a calibration RMS error of 0.54±1.40 mm which is promising for surgical applications. We conclude this paper by showcasing two clinical applications. One is a markerless registration application, the other is an RGB-D camera augmented mobile C-arm visualization.