Interior permanent magnet (IPM) machines are widely adopted as traction motors in electric vehicles (EVs), with their vibration and noise issues obtaining more and more research attentions. The radial magnetic force harmonics applied on the stator, are the main factor of electromagnetic (EM) vibration in electric machines. Relationship between open-circuit magnetic field and corresponding EM force behavior of IPM machines have been extensively investigated and well revealed, while the contribution of armature reaction field to EM force is still not well understood. In this paper, onload magnetic field harmonics in a typical 160kW, 48-slot, 8-pole IPM motor, are analyzed by analytical method and finite element tools, to investigate their contributions to radial EM force harmonics, respectively. Frozen permeability (FP) technique is utilized to extract and highlight the influence of armature reaction on EM force. The zeroth-mode EM force harmonics of three typical peak operating conditions of the motor are analyzed and corresponding radial acceleration responses are calculated. It is interesting to find that the armature field not only can increase the amplitude of the zeroth-mode vibration via generating EM force harmonics with existing time order compared with that of PM field, but also inducing additional zeroth-mode force harmonics with different time orders. The analyses results indicate clearly that armature reaction is more influential than open-circuit PM field in terms of inducing 0th mode EM vibration under peak operations with high current loading in IPM traction motors.