With the viewpoint of biomechanics in theory, this paper analyses why children are liable to have fracture in supracondyle of humerus as a result of falling to the ground with the upper limbs propping up the body while adults usually have fracture in the middle or upper part of humerus under the same condition. The focus of discussion is the change of front-inclination in the lower end of humerus in the process of bone maturity from childhood to adulthood.