A method to improve the accuracy of 3-dimensional coordinate measuring machine and a convenient method to test its measuring accuracy were studied. Error vector of the measuring position was defined in the machine, and the relation between it and volumetric error obtained by the double ball bar test was theoretically analyzed. Double ball bar test equipment was fabricated and its performance was strictly examined. The results show that it has sufficient accuracy and conveniency for machine shop use to evaluate the accuracy of the 3-dimensional coordinate measuring machine. The volumetric error obtained by double ball bar test was compensated using error vector which was reduced from the measured straight motion errors of the machine. It was found that the volumetric error obtained by the double ball bar test was significantly decreased by the compensation.