This paper discusses the 3-point error separating technique for on-line measurement of straight line error motion of an ultra-precision lathe and straightness of the generatrix of turned works. The accuracy of the measuring system is obtained about 土0.078μm (土3σ). Furthermore，a closed loop，micro-computer control system is built up to process the error data and excite a servo-mechanism for Compen sating the error motion，and the straightness of the turned work is increased to 0.5μm/160mm，that is，Compensating increases straightness by 50%，at least than before compensating.