Objective: For skull base tumors, Gamma knife surgery (GKS) is effective to control them. However, it is not possible to treat large tumors which compressed brain stem with GKS alone. Needless to say that microsurgical resection must be considered. We propose that precise preoperative treatment design and intraoperative image supporting system are crucial for them. We developed a system by incorporating a technique for Gamma Knife surgery to design a treatment plan before and during surgery. Methods: Magnetic resonance imaging was performed without a stereotactic frame before treatment. Image data was imported to Leksell GammaPlan (LGP) to obtain 3D localization in order to understand anatomical relationship between the tumor and surrounding vital structures. Results: For skull base meningiomas, the dura maters from which tumors arose were identified on LGP. Certain parts of the tumor should be spared by GKS, because there are the most important vital structures such as cranial nerves. Other parts should be then surgically removed as much as possible to be the best decompression in the tumor. For acoustic tumors, we identified facial and acoustic nerves as much as possible using this simulation technique. It can provide the best knowledge of anatomical relationship to neurosurgeons before and during surgery. Most of patients would not have experience with neurological deficits. Conclusions: Sophisticated 3D computerized image data in LGP enabled us to perform surgery just as planned preoperatively. This new and simple treatment system will be a precursor of future surgical treatment strategies.