For Clarifying the mechanism of tool and machine handle grasping, sensory tests to determine the optimum grasping diameter were performed on 43 people aged seven to fifty-three years. It was confirmed that the sensory evaluation for determining the optimum grasping diameter was difficult for children under age ten due to insufficient growth of the skeletal structure. The optimum grasping diameter was thus determined for persons above age ten. The optimum grasping diameter was found to depend on the hand size, strongly on the length of the hand but only weakly on the hand breadth or fist size. The optimum grasping diameters for the males were 30-40 mm, about 10% larger than those for the females. It was also found that a cylinder could be grasped well by a human hand if all the finger tips and the entire palm of the hand were in contact with it.