Objective: To observe the process of tumor response to interstitial brachytherapy of intracranial germinomas during and immediately after the therapy. Methods: Fractionated brachytherapy using 192Ir was used in 13 patients with intracranial germinomas. The average age at diagnosis was 14.3 years (range 5 - 27 years). The frequency of fractionation had been increased stepwise from 3 fractionations within 5 d in the past to 26 - 30 fractionations in 15 - 20 d at present, Results: Of 9 pineal tumors, the average reduction of tumor volume was 73.2% at a cumulated radiation dose of 20 Gy and85.3 % at 30 Gy, respectively. In 4 cases of basal ganglia and thalamus areas tumors, the reduction of tumor volume was from an average 56.1% at a cumulated radiation dose of 30 Gy to 75.2% at the end of therapy. That intratumoral hemorrhage was absorbed quickly accompanying tumor regression was demonstrated in all but one patient; 6 patients died; 7 patients were still in follow-up without any evidence of tumor recurrence. Conclusion: Interstitial brachytherapy could be given as the first optional therapy for intraeranial germinomas. The histological diagnosis of germinomas can be verified by stereotactic biopsy simultaneously. Germinoma is one of the sensitive tumors to interstitial brachytberapy. The radiation injury to surrounding brain tissue could be reduced by using a conformal dynamic brachytherapy.