We show how to compute the extrinsic parameters of a video camera from the optical flow measured in consecutive video frames. We assume that the camera is mounted on a car or a robot which move forward and sideways on a flat area to acquire the images used for the calibration. The vanishing points of the optical flow lines provide enough information to compute the camera rotation matrix. We also show how the lines on the ground together with the vehicle's velocity provide enough iinformation to compute the camera position.