Abstract: Aim To discuss the fundamental principle of error separation method for roundness and spindle error motion measurements with multiprobe and the propagation patterns of the measurement errors for all harmonics of roundness as well as the relation between roundness measurement accuracy and the number of the probes. Methods\ Under the prerequisite for error separation, extra probes were used to introduce redundant measurement equations by which the measurement errors caused by the reading errors and misalignments of the probes were effectively decreased for all the harmonics of roundness. The relation between the number of the probes and roundness measurement accuracy was analyzed according to error theory and investigated by experiments. Results and Conclusion\ Three point method can not improve roundness measurement accuracy further. The measurement error caused by the reading errors and the misalignments of the probes can be decreased effectively by using more than three probes to introduce redundant measurement information, the accuracy for roundness measured by four point method is improved significantly. However, the roundness measurement accuracy can only improve to a very limited extent by using more than four probes in comparison with that of four point method.