The direct calibration of MEMS springs made of single crystal silicon was carried out using a mechanical force measurement tool. The spring device for calibration fabricated from silicon on insulator (SOI) wafer contains four folded-beam springs. The spring constants of different-length devices were directly measured using electromagnetic force-feedback balance, whose force resolution was about 0.2 muN. The spring constants of the two types of folded-beam springs, whose designed constant was 0.3 and 0.7 N/m, were 0.18 and 0.57 N/m, respectively. The spring constants were successfully calibrated, which shows that this tool can calibrate various kinds of MEMS flexible structures.