The purpose of this study was to clarify the role of arm swing in running motion through a comparison between arm swing restricted and normal running. Eight male university long distance runners participated in this study. They ran with the maximum speed in different conditions (a: normal running (arm free), b: one side arm fixed (right and left), and c : both arms fixed). Their running motions were recorded using three high-speed video cameras at 250 fps from which the three-dimensional kinematic data were obtained. In the arm fixed conditions (b and c), the running speed decreased significantly and the motion of trunk and lower limb were altered markedly compared with those of the normal running. For the trunk motion (within the horizontal plane), the shoulder rotation angular velocity increased and the hip rotation angular velocity decreased significantly. Moreover, a significant decrease of the hip extension angular velocity and the whole leg angular velocity during the support phase were clarified. It was considered that both the horizontal rotational motion of the upper body and its counter rotational motion of the lower body were altered as compensations when the fundamental arm action was asymmetrically or fully restricted. These altered trunk motions likely forced runners to adjust their leg motion, thereby decreasing their running velocity.