Abstract: Aim To improve the accuracy of measuring roundness and spindle mo tion error with three point method. Methods The effects of the reading error and the misalignment of the probes on the accuracy of measuring and all the harmonics of roundness were analyzed; The strategy and the algorithm for optimizing the parameters of the measuring system were proposed; The distribution and propagation patterns of the measured errors for all the harmonics of roundness were studied in detail; A software for correcting the errors caused by the measurement error of the angle positions of the probes and the sensitivity difference between three probes was presented. Results\ The maximum error of measuring the harmonics of roundness was reduced by 67% in comparison with that by the traditional three point method. Conclusion The accuracy of roundness measured by the improved three point method gets a distinct improvement.