In the hobbing cutting process,with the blade rows cutting in and cutting out,the cutting force increases gradually,then gradually becomes small,a periodic vibration force will be produced between the workpiece and tool,which produces a relative displacement between the blade and workpiece surface,influencing the geometry precision of gears in hobbing.Relationship between the structure vibration of machine tool and machining precision of gear was studied.The theoretic model that the influence of the relative displacement on the geometrical precision of gears was built,the tooth profile error produced by the relative displacement was deduced quantificationally.By a large number of experiments,the relative displacements between the hob and workpiece surface and the gear tooth profile error were measured online precisely.The new method of tooth profile error subtraction operation was used to analyze and compare two groups of experimental data.Results show that the proposed new method works accurately on data.The experimental data well verify the correctness of the theoretic model built,and error is less than 2%.