The three-dimensional vibration behavior of a rotating crankshaft system under firing conditions can be analyzed either by using (1) a stationary coordinate system attached to the engine body, or (2) a rotating coordinate system attached to the crankshaft body. For the former, since a crankshaft body has anisotropic properties, e. g., each crank-throw has different bending stiffness in the two orthogonal planes, time-variable properties can be introduced in the equations of motion. Therefore, the solution can only be obtained by a numerical method in the time domain. However, for the latter, the equations of motion can be derived as a system of time-invariable properties, and the vibration behavior of the crankshaft system can be calculated directly in the frequacy domain. In this paper, for a rotating coordinate system, (1) we derived the time-invariable characteristic matrices of multidegrees of freedom for the crankshaft system, and (2) the forced vibration behavior of the crankshaft system under firing conditions was calculated in the frequency domain. The validity of the approach was confirmed from the calculated and experimental results.