A safe robot arm can be achieved by either passive or active compliance. The passive compliance systems composed of purely mechanical elements often provide faster and more reliable responses for dynamic collision than the active ones involving sensors and actuators. Since both positioning accuracy and collision safety are important, a robot arm should exhibit very low stiffness when subjected to the collision force greater than the one causing injury to humans, but maintain very high stiffness otherwise. To implement these requirements, a novel safe link mechanism (SLM), which consists of linear springs, a double-slider mechanism and shock absorbing modules, is proposed in this research. The main contribution of SLM lies in its variable stiffness capability implemented only by passive mechanical elements. Various experiments for static and dynamic collision show that the stiffness of SLM is kept very high for the external force less than the critical impact force, but it drops abruptly as the external force exceeds the critical force, thus guaranteeing the collision safety. Furthermore, the critical impact force can be set to any value depending on the applications.