novel time-domain error separation method which can reconstruct straightness profile of workpiece exactly for on-machine measurement has successfully been developed. The proposed method is based on difference measurement and can use two or three displacement probes. It possesses following characteristics: (i) adapting to long or short workpiece, (ii) assuming no prior knowledge, (iii) employing large shears, (iv) needing no accurate zero-adjustment of probes, and (v) reconstructing various surfaces including smooth, non-smooth, periodic and non-periodic profiles with no theoretical error. The shortest length which can be reconstructed exactly has been calculated in time-domain method. The theoretical analysis justifies the effectiveness of this method.