Background: In this study, we retrospectively reviewed the treatment outcome of 63 patients with T1 early glottic cancer treated with RT alone to determine the treatment outcome and the prognostic factors affecting local control. Materials and Methods: All patients were treated by 6 MV photons with conventional bilateral fields up to a median dose of 66 Gy in 33 fractions. Results: The 5-year local control rate and overall survival were 77.7% and 93.1%, respectively. The total radiation dose with a cut-off value of 66 Gy was a significant prognostic factor for local control. The 5-year local control rate was 54.5% in patients treated with less than 66 Gy compared to 85.7% in patients treated with 66 Gy or higher dose (p = 0.014). In subgroup analysis, in patients who received 66 Gy or higher doses, all recurrences developed in whose overall treatment time was 49 days or longer, although the statistical significance was marginal (p = 0.066). Conclusion: This study showed that a total dose of 66 Gy or higher is required for the treatment of T1 glottic cancer, and delivering the total dose within 49 days seems important for local control.