The medical records of 133 patients with Stage I tongue cancer treated by definitive radiotherapy between 1966 and 2001 were reviewed. Overall survival rate (OS), progression free survival rate (PFS), and survival rate after recurrence were calculated according to the Kaplan-Meier method. We investigated prognostic factors for local control and risk factors of late neck LN metastasis. The 5-year OS was 81.8% and the 5-year PFS was 67.2%. The 5-year OS after local recurrence was 100% by salvage operation, and that after neck LN metastasis was 40.7% despite radical neck dissection. Tumor thickness over 5 mm and treatment without interstitial irradiation were prognostic factors for local control. Tumor diameter over 15 mm and tumor thickness over 5 mm were risk factors of late neck LN metastasis. We should consider prophylactic treatment for neck LN for high risk patients with Stage I tongue cancer in order to improve treatment results further.