Prostate cancer is the most common cancer diagnosed in men in the United States. Prostate brachytherapy - an advanced radiotherapy cancer treatment in which radioactive seeds are placed directly in and around the prostate tissues - is complicated by tissue forces that tend to deflect the treatment needles from their intended positions. Medical simulation has proven useful as a technique to enhance surgeons’ training and procedure rehearsal. Therefore we are developing a brachytherapy simulation system that includes visual display and haptic feedback, with which a user can see and feel the procedure of the therapy. In this paper, we describe a deformable needle insertion simulation that is a part of our brachytherapy simulation system.