Abstract: A method for determining the precision of gears includes steps of providing a gear pair; performing a single flank test for the gear pair, to generate a testing signal graph; decomposing the testing signal graph into a plurality of intrinsic-mode-function graphs; selecting a first function graph and a second function graph from the intrinsic-mode-function graphs; measuring the amplitude of vibration of the first function graph, to get a profile error of gear; combining the first and second function graphs to form a graph of function combination; calculating an adjacent pitch error and an accumulated pitch error by means of the graph of function combination; and defining the gear precision for one of the gear pair according to the profile error of gear, the adjacent pitch error and the accumulated pitch error.