The stereotactic radiosurgery using ionizing radiation of high energy is a technique for exadicating intracranial small tumors, which are inaccessible or unsuitable for open surgical technique. For such a small field radiosurgery, TLD or film dosimetry is essential. The three dimensional dose planning of radiosurgery was performed with dose planning computer system (Therac 2300). The target dose distribution and its error according to patient position were discussed. And were measured of circular cone which specially designed in our Hospital. The position error of Rando Phantom compared with CT were 0.4 mm in the AP-LAT section and in the AP-VERT section, 1.0 mm in the AP-VERT 45°section. The ratio of accuracy of the gantry and couch rotation were 1.5 mm diameter for central axis of 18MeV linear accelerator. Our study suggested that radiosurgery of small field in out department will be appropriate for clinical application.