To deal with the interactive separation of region of interests (ROI) in the process of three-dimensional (3D) visualization, a half-automatic adjusting method based on the optical model for volume rendering is introduced. According to the optical transfer-function model and the intensity distribution of 3D data, the interactive model is described by adjusting the RGB color and transparency values respectively with the help of the 3D gradient field. In order to make the method more effective, the interactive model and the texture feature of modern graphics are both applied to accelerate the process. The experimental results show that an optimal separating effect is achieved by adjusting the corresponding parameters in the transfer function model in real time. And the frame rate is up to 30 f/s in the interactive process. With this method, the optimal parameters are obtained in the interactive volume visualization.