A new type of measuring instrument which is used to measure lead errors of ballscrews has been developed. The principle of the inStrument,the system strucrure,the computer inter-face. The analyses of the instrument error and the experimental results are presented, The principleof comparing synchronous displacement is used to achieve absolute measurement. The errors correc-tion technique is adopted to correct the errors of the instrument so that the high measuring accuracycan be botained.