The aim of the present study was to retrospectively investigate the risk factors of local failure for T1 glottic carcinoma irradiated with a prescription dose of 66 Gy. Between July 2006 and December 2017, 64 patients with T1 glottic squamous cell carcinoma treated with 66 Gy/33 fractions were analyzed for risk factors of local failure. The sex, age, performance status, T stage, overall treatment time, anterior commissure involvement, smoking status during/after treatment, histological tumor grade and pretreatment hemoglobin level were investigated. The maximum, mean and minimum doses, and the homogeneity index for the glottic larynx were calculated for dosimetric risk factors of local failure. The median follow-up duration was 51 months. Local failure was observed in 6 patients (9.5%). Among all risk factors, only the minimum dose to the glottic larynx was found to be significantly associated with local failure (P=0.025). The 5-year local control rates for a minimum dose to the glottic larynx of <65 and ≥65 Gy were 79 and 95%, respectively, with a statistically significant difference (P=0.015). No patients exhibited grade ≥3 late adverse effects. The minimum dose to the glottic larynx was the only factor significantly associated with local failure. Thus, local control of T1 glottic carcinoma may improve with a minimum dose of ≥65 Gy to the glottic larynx. In conclusion, radiotherapy with a minimum prescription dose of ≥65 Gy to the glottic larynx appears to be safe and achieves a high local control rate for T1 glottic carcinoma.