Abstract: This paper describes a free hand navigation system for high-dose-rate brachytherapy. The whole system pipeline consists of four steps, namely, pre-processing, pre-operational planning, intra-operational navigation and post-operational planning. During pre-processing, the user can decide the positions of operational catheters and draw contouring of the target volumes interactively. For intra-operational free hand navigation, the catheters are tracked by 6-DOF magnetic-tracking technology instantly and accurately. At the same time the high-quality volume visualization techniques guide the users implant the catheters to pre-defined poses. Thereafter the dose calculation and optimization are carried out after the implantations of catheters are fulfilled. Finally, the radioactive sources are inserted and dwelled by using the after loader to implement the radiation. According to our preliminary experiments, this system achieves encouraging 2-3 times accuracy improvement compared to the trial-and-error manual implantation method. Moreover, the operational time is saved dramatically which decreases the painfulness of the patients.