Neoadjuvant chemoradiotherapy (CRT) has been the standard therapy for local advanced rectal cancer. Pathological complete response (pCR) is an important prognostic factor for local control and survival. A high intensity CRT increases not only the pCR rate, but also toxicity, especially diarrhea. Compared with traditional RT technique, intensity-modified radiation therapy (IMRT) can decrease the toxicity of diarrhea because of low volume of high dose for small bowel. Therefore, IMRT technique provides an opportunity to improve the dose intensity of neoadjuvant CRT. The investigators hypothesize that a higher treatment dose induces a high rate of pCR and design a two-arm trial. in this trial, low intensity CRT includes the whole pelvic irradiation of 50Gy together with Oxaliplatin and Capecitabine weekly. While in high intensity group, additional concomitant 5Gy for primary tumor and a cycle of Xelox are prescribed. All patients will receive a total mesorectal excision (TME) 8 weeks after CRT.

Inclusion Criteria:

• Patients with rectal adenocarcinoma
• Clinical staged T3/4 or any node-positive disease
• Age: 18-75 years
• Karnofsky Performance Status > 80
• Adequate bone marrow reserve, renal and hepatic functions
• Without previous antitumoural chemotherapy
• No evidence of metastatic disease
• Written informed consent before randomization

Exclusion Criteria:

• Previous pelvis radiotherapy.
• Previous antitumoural chemotherapy
• Clinically significant internal disease