High-dose-rate interstitial brachytherapy (HDR-ISBT) is relatively rarely applied for the head and neck cancer. However, its dose distribution is more confined than intensity modulated radiation therapy (IMRT) and can deliver higher dose while sparing surrounding normal tissues. In this case report, the effectiveness of HDR-ISBT as a boost following IMRT for post-operative recurrent oropharyngeal cancer patient was indicated. A 73-year-old male who developed local recurrence after surgery for oropharyngeal squamous cell carcinoma. Salvage IMRT up to 70 Gy concurrent with weekly cetuximab was planned. However, CT taken at 60 Gy found a residual tumor, then, boost HDR-ISBT was proposed. 1 week after 60 Gy of IMRT, HDR-ISBT, 12 Gy in 2 fractions, was delivered under local anesthesia. MRI taken 2 months after HDR-ISBT showed no residual tumor. It was demonstrated that boost HDR-ISBT following IMRT for local recurrence of oropharyngeal cancer was performed safely and showed favorable efficacy.