FSTRP (Fine Sequentia1-Three-Points)-a new method for on-line measurement of the straightness of precision machine tools and machined workpiece is proposed in this paper. FSTRP uses three sensors and chooses suitab1e distances between every two adjacent sensors，we can obtain smaller samp1ing interva1 than before. The improved a1gorithm of FSTRP is derived and its flow diagram is a1so presented in it. The results of computer simu1ation and on-line measurement experiment on precision 1athe are a1so given. It can be shown that this method is feasib1e and perfect.