An error measuring method of gear includes: performing the single gear flank testing to the gear pair in order to produce a testing signal graph; the operation unit decomposing the testing signal graph by Empirical Mode Decomposition (EMD) to generate a plurality of intrinsic-mode-function graphs (IMFs), the intrinsic-mode-function graph (IMF) having a first function graph with a first range and a second function graph with a second range, the first range is greater than the second range; the operation unit selecting a plurality of third function graph from the intrinsic-mode-function graphs (IMFs), wherein the third range is greater than the second range but not greater than the first range; the operation unit combining the third function graphs to produce a graph of superposing function; the operation unit computing out a plurality of single pitch error, a plurality of adjacent pitch error and an accumulated pitch error.