Abstract: In this paper, the coupling relationships between three-dimensional vibration of the crankshaft and surface vibration of the crankcase were studied by using newly designed torsion/axial/bending vibration testing device and coherence analysis. The result shows that surface vibration of the crankcase is induced by 3D vibration of the crankshaft. Torsional vibration can induce surface vibration in some frequencies, and the frequencies of the surface vibration are commonly twice as much as that of the torsional vibration; The frequencies of axial vibration and bending vibration are the same as that of surface vibration. The coherent coefficients between axial vibration and surface vibration are not high except some frequencies and bending vibration is the main exciting source of surface vibration.