In this work, we studied human head motions during upper extremity motor task, conducted in functional magnetic resonance imaging (fMRI) studies. The experiments were performed on a mock up of the MRI bench. Three kinds of linear movements in the caudal-cranial, left-right and anterior-posterior directions, a reaching movement as well as single joint movements at shoulder, elbow and wrist, were investigated at three different speeds with the right arm. Optical markers tracked by a high-precision motion tracking system were used to detect head and arm movements. For each task, the head moved principally in the direction of right-caudal-posterior to left-cranial-anterior, parallel to the line from the right shoulder joint to the head. No principal rotation axis was observed. The translational and rotational head motions were 1.7 ~ 8.4 mm and 0.15deg ~ 0.96deg, respectively. We then applied a special head fixation to stabilize the head, and repeated all the experiments. The translational and rotational head motions were significantly suppressed by 78% and 73%, to the range of 0.3 ~ 2.4 mm and 0.04deg ~ 0.39deg.