Abstract: In order to explore methods for the error diagnosis of extension strut in the variable axes numerical control machine tool(VNCMT), the vector method based on path circle is put forward according to the principle of error diagnosis of conventional NC machine tools, and effects of extension struts on the platform is studied. Using the measured error of the extension struts, both the graphical method and numerical method were adopted, then vector method proved to be correct. This study offers the possibility for the error diagnosis of extension strut of such machine tools through the ball bar.