Natural frequencies, natural modes and dynamic response of a crank shaft of an internal combustion engine are analyzed by the reduced impedance method the author proposed. A crank shaft is divided into the straight shaft parts and the arm ones. The dynamic behavior of the straight shaft part is expressed first by the transfer matrix, which is transformed into the reduced impedance matrix. Meanwhile the arm part is divided into the tetrahedral elements by the finite element method, and the degrees of freedom of their boundary surface parts are reduced to six. The equation of motion of the arm part is reduced to form a reduced impedance matrix with respect to the boundary parts. The reduced impedance matrices of all components are combined to give a global equation of motion of the crank shaft, and this equation is solved to get natural frequencies, natural modes and dynamic response. The calculated results agree well with the experimental ones.