Following the previous report which proposed a new calculation model for crank shaftings of the long stroke marine diesel engine and an evaluation method for the stiffness of crank throws, a calculation method for the coupled axial-torsional forced vibration is presented in this report. In the method, conversion of the crank pin force into the equivalent axial force is newly introduced. Moreover, accurate axial-torsional vibration measurements were carried out during both shop and sea trials. Minute analysis on the test results revealed the mechanism of the vibration. Calculation results by the new method show fairly good agreement with the measured ones only except for higher harmonics oscillation at the resonance of 1-node torsional vibration, which is recognized to be caused by non-linearity of the thrust block stiffness.