We proposed an eye gaze tracking system under natural head movements. The system consists of one CCD(charge-coupled device) camera and two front-surface mirrors. The mirrors rotate to follow head movements in order to keep the eye within the view of the camera. However, the mirror controller cannot guarantee the fast head movements, because the frame rate is generally 30Hz. To overcome this problem, we applied Kalman filter to estimate next eye position from the current eye image. In the results, our system allowed the subjects head to move 60cm horizontally and 40cm vertically, with the head movement speed about 55cm/sec and 45cm/sec, respectively. And spatial gate resolutions were about 4.5 degree and 5.0 degree, respectively, and the gaze estimation accuracy was 92% under natural head movements.