Direct stiffness calibration of microelectromechanical-system (MEMS) springs made of single-crystal silicon was performed using a previously developed mechanical force measurement tool. The spring devices used for calibration were fabricated from silicon-on-insulator wafers and contained one or four folded-beam springs. The spring constants were directly measured using an electromagnetic force-feedback balance with a force resolution of about 0.2 mu N. The average measured spring constant of a folded-beam spring with a designed constant of 0.7 N/m was 0.58 N/m. This successful calibration shows that this tool can be used to calibrate various kinds of MEMS flexible structures. (C) 2007 Elsevier B.V. All rights reserved.