Robotic technology has become more interested field to safely interact with the human. Therefore, there is a need to continuously improve the mathematical models and flexibility of robot arm when subjected to a collision force in order to reduce environmental impact, but maintain very high stiffness otherwise. To implement these requirements, adjustable compliant mechanism (ACM), which consists of flat plate spring (FPS), two thin disks, ball bearing and stepper motor, is proposed in this research. The ACM has advantages of variable stiffness which can be achieved only by passive mechanical elements. The theoretical relation between spring stiffness and rotational angle was derived for rectangular cross section. Comparison was made between three types of materials used for FPS which were AISI Steel 4340, Brass and Aluminum 6061 T6. The steel spring exhibited the best flexibility and good rigidity as well. Several of impact forces showed the variable stiffness of the FPS, but an abrupt drops in the stiffness when rotational angle was 90° under various impact forces. Furthermore, the stiffness and impact force can be set to any value depending on the application.