Calibration of the spring constants of atomic force microscopy (AFM) cantilevers is one of the issues in biomechanics and nanomechanies for quantified force metrology at pieo- or nano Newton level. In this paper, we present an AFM cantilever calibration system: the Nano Force Calibrator (NFC), which consists of a precision balance and a one-dimensional stage. Three types of AFM cantilevers (contact and tapping mode) with different shapes (beam and V) and spring constants (42, 1, 0.06 N $m^{-1}$) are investigated using the NFC. The calibration results show that the NFC can calibrate the micro cantilevers ranging from 0.01 ${sim}$ 100 N $m^{-1}$ with relative uncertainties of less than 2%.