In the present study, we examined the appropriate schedule of S-1 medication in the combination with radiation by investigating the safety, the clinical efficacy, and antitumor effects on tumors in nude mice. In the patients with oral squamous cell carcinoma (OSCC), S-1 was given orally according to a 4-week application followed by 2-week rest regimen (4-week regimen), or a 2-week application followed by a 1-week rest regimen (2-week regimen). Radiation was given (2 Gy/day; 5 days/week) for a total of 60 Gy. In nude mouse models, human oral cancer cell lines were used as subcutaneous xenografts in nude mice. The mice were treated by S-1 (10 mg/kg) and radiation (1 Gy) with a 4-week regimen or a 2-week regimen. Apoptotic cells were detected by TUNEL method. In the patients with OSCC, the response rate with the 4-week regimen was 100% and the response rate with the 2-week regimen was 92.3%. However, a high frequency of adverse effect was found in the 4-week regimen when compared to the 2-week regimen. Grade 3 toxicity of leukopenia, neutropenia and stomatitis were seen in 3 cases, grade 3 toxicity of anorexia and nausea were seen in 2 cases, and grade 3 toxicity of decrease of hemoglobin level, heartburn/dyspepsia and increase of bilirubin level were seen in a case of the 4-week regimen. On the other hand, grade 3 toxicity of stomatitis, anorexia, nausea, heartburn/dyspepsia and increase of bilirubin level were seen in a case of the 2-week regimen. In nude mouse models, the 2-week regimen was more effective than the 4-week regimen. In addition, significant increase in the percentage of apoptotic cells was observed in the tumors treated with the 4-week regimen when compared with the tumors treated with the 2-week regimen. No loss of body weight was observed in mice treated with the 2-week regimen during the experimental period. These results suggested that the 2-week regimen might reduce adverse effects, and enhance therapeutic effects compared to the 4-week regimen. Briefly, this 2-week regimen may be a useful concurrent chemo-radiotherapy improving the quality of life (QOL) of patients with OSCC.