In order to improve the measurement accuracy of the optical sta ndard ball′s shape errors,a roundness error separation method based on single transposition characteristic a ngle is proposed.The method is based on strict mathematical theory and uses the characteristic angle in the sha pe of the reference,and it can completely avoid all the blind spots which may produce a harmonic suppression.Through one t ransposition,the method makes Fouier transformation on the pre-and post-sampl ing data,and separates the optical standard ball′s roundness errors and radial rotation errors in frequency domain.And then it makes the separated data transfo rmed to time domain,and makes no loss of the separated signal before the filter.The modeling process introduces no pri nciple error.The separation of the radial rotation error and the roundness error of the optical stan dard ball is achieved completely. Experimental show that when the circle sampling points are 1 024and the translocation characteristic angle is 21.09′, the roundness error contains all frequency harmonics and no harmonic suppression after separation,indicating a strict separation.The method can be used for ultra-precision optical standard ball roundness measurement and establishing an idea l benchmark / standard circle measurement system.