This paper presents the design of a compliant constant force output gripper mechanism. The function of constant force output is achieved by using the negative stiffness effect of a buckled fixed-guided beam. One advantage is that it can eliminate the needs of complicated force-displacement combined control algorithm. Details of the nonlinear design process have been demonstrated. ANSYS APDL and MATLAB are used to solve this nonlinear problem. The structural design of the gripper is performed based on the theoretical model. An experimental study is carried out to verify the theoretical model. A force sensor and a displacement sensor are used to test the performance of the constant force output in the experiments. Results shows that the gripper can provide 1.1 N near constant force output in 200 μm range.