This paper addresses the problem of determining the absolute force constant of Atomic Force Microscope cantilevers. In the method presented, the cantilever under test is deflected against a reference cantilever of known spring constant. The relative deflection of the two cantilevers is related to their spring constants. The novelty of our approach is in the use of a micromachined reference cantilever of a precisely controlled force constant. Preliminary results show that our method is capable of measuring the force constant of cantilevers in the range of 0.1 to 10 N/m with an accuracy of better than 20%. The error is dominated by the non-linear effects in the force versus distance curves used for the measurement.