A phase I/II study of postoperative chemoradiotherapy with weekly cisplatin for head and neck squamous cell carcinoma was conducted. The eligiblity of patients were the high risk features, i. e., multiple lymph nodes metastasis(2 or more), extracapusular extension of nodal disease(ECE), or the presence of tumor at the surgical section margins(at 5mm or less). The recommended dose of CDDP in a phase I study was 30mg/m2. We performed a phase II study to assess toxicity and tolerability. We assessed 13 patients, 10 of whom were enrolled in a phase II study, and 3 patients in phase I were given the RD. Acute adverse events were rather mild, including grade 1-2 anemia(50%), mucositis(43%)and nausea/vomiting(43%). One patient required administration of CDDP to be discontinued due to grade 1 renal toxicity. The compliance rate was markedly high(85%: 11/13 patients). We consider weekly CDDP of 30mg/m2 to be a safe regimen in the setting of postoperative adjuvant chemoradiotherapy.