Phase I Study of CCRT as Adjuvant Treatment for Stage II/III Operable Rectal Cancer.

Inclusion Criteria: * R0 resection of histologically proved stage II/III rectal cancer; * 18-75 years old; * No previous radiotherapy or chemotherapy for rectal cancer; * Performance status of ECOG 0,1; * Adequate organ function defined as below: i. WBC ≥ 4,000/mm\^3 ii. ANC ≥ 1,500/mm\^3 iii. Hemoglobin ≥ 10g/dL iv. Platelet ≥ 100,000/m\^3 v. Total bilirubin ≤ 1.5ULN vi. AST/ALT ≤ 1.5ULN vii. Serum creatinine ≤ 1.5ULN or creatinine clearance rate ≥ 60ml/min、Urea nitrogen ≤ 1.5ULN viii. Protein in urine dipstick test\<1+; if the test result \>1+ ,total protein in urea must \<500mg within 24 hours * Able to receive oral administration * Informed consent Exclusion Criteria: * Hypersensitive to S-1 or its excipients * Pulmonary fibrosis or interstitial pneumonitis found within 28 days prior to registration * Significant co-morbid medical conditions, including, but not limited to, heart failure, renal failure, hepatic failure, hemorrhagic peptic ulcer, mechanical or paralytic ileus, or poorly controlled diabetes * Received any investigational drug or anti-cancer agent * Pregnant or lactating female or pregnancy test positive * Severe mental disorder * Judged ineligible by physicians for participation in the study due to safety concern.