A computer aided brain surgery system using virtual reality techniques is developed. This system is aimed to be used to support the surgeons' decision for the operational strategies, or used in the training of medical students to learn how to operate the brain surgery for the patients. In constructing the whole system, high speed graphical computer is equipped. For input 3D data from MRI or CT are used to display 3D images, HMD (head-mounted display) and 3D CRT display with glass are equipped. To improve the 3D images, colors and optical properties of the voxel are refined.