In 40 Gb/s systems where low polarization-mode dispersion(PMD) fibers are used,the PMD values are usually very low,and most of them are first-order PMD.Based on the 40 Gb/s NSFC Net,we set up a PMD compensator consisting of an electrical polarization controller(PC),a polarization maintain fiber(PMF),a degree of polarization(DOP) detector and a feedback control module.This compensator has a simple structure and is very sensitive,it can compensate up to 20 ps first-order PMD adaptively and the average searching time is 2 ms.