For the purpose of getting better therapeutic ratio of radiation therapy, an intra-operative irradiation was scheduled and attempted. The advantages of this modality are 1) only the target area is irradiated under the direct vision, 2) unnecessary area is not irradiated due to the selected field and selected electron beam energy, and 3) repeated fractionated irradiation are not necessary and an intensive post-operative chemotherapy can be started earlier. Electron beams of betatron are used according to the necessary depth of tumor volume: usually 8 or 10 MeV are used. The irradiation fields changed by the plastic treatment cones 4 cm to 12 cm in diameter according to the necessary tumor field. One of the biggest problems in an actual attempt of intraoperative irradiation was the determination of a single radiation dose. Actual time-dose relationship of the cured cases of Wilms' tumor and neuroblastoma was investigated by world literature. The single doses for 2 years old patients were calculated from Ellis' formula. They were 9.4-11.8 Gy for Wilms' tumor and 6.5-12.2 Gy for neuroblastoma. Consequently, the single dose was started from 10 Gy and increased up to 15 Gy at the present time. External irradiation was scheduled for supplement if necessary. The details of irradiation on 5 cases were discussed. The clinical evaluation of this treatment method will take time, and many problems remain to be solved clinically and experimentally, such as the radiation effect and side effect of a large single dose.