In this study, a novel constant-force mechanism (CFM) is introduced. A design method is proposed to employ the compliant spatial slider-crank mechanism as CFM. Design geometry and initial conditions are presented for the selected configurations of the compliant constant-force mechanism (CCFM). Design parameters are nondimensionalized. Parameters of the PRBM that result in a CFM are determined via minimizing the variation in the output force over the given input displacement. A generalized solution method is derived to determine optimum results and some design charts are presented. In order to verify feasibility of the design procedure, a real model is built and it is shown that the experimental results are consistent with the theoretical approach. (C) 2013 Elsevier Ltd. All rights reserved.