The purpose of this study was to investigate the effect of arm swing on the generation of twisting from somersault. By using a human body model composed of three geometric solids, mechanism of generation of twisting during a somersault was confirmed theoretically. Then, quantitative calculation was done based on the model. In this model a performer initially took a symmetrical position with both arms above the head, and then swung down one arm laterally. To examine the validation of the model, several twist-somersaults were analyzed by a 3-dimensional cinematography (DLT method) and compared with the model. Two male proficient gymnasts were selected as subjects and were asked to perform forward twist-somersaults of 1/2, 1 and 3/2 revolutions from a vaulting horse. The results were summarized as follows: 1) The angular velocities of the twist and somersault, moment of inertia, and the tilting of a body's principal axis of inertia from its original position calculated from the model were compared with the measured data. The mean relative errors between the data from simulation model and the measured were less than 4.1%. 2) An asymmetrical arm swing could generate a twist about the longitudinal axis of the body during a somersault. Large angular velocity of the somersault before the change in the form and large swing angle of arm were effective for the generation of twisting. 3) The direction of the twist depended upon direcitons of the initial somersault and the arm swing.