Abstract: This invention relates to a gear error measuring method and a measuring machine thereof through meshing and separating. The error is measured and taken during the meshing engagement of a specific master gear with the measured gear. As the specific master gear with its teeth arranged according to a specific rule and a device for fixing the unified starting position on the machine are adopted, various individual errors, composite errors, three-dimensional flank form error graphs, integrated error graphs and contact morphological graphs can be measured and obtained in a measuring cycle, furthermore, the interrelationship among different individual errors and between the individual error and the composite error can be shown in the measuring result. This invention is especially applicable to the error measurement of bevel gears and hypoid gears where different errors will hardly be measured by the existing measuring method at present.