Abstract: Objective To investigate the changes of apparent diffusion coefficient（ADC） of advanced colorectal cancer pre-and post-chemotherapy，as well as the correlation of ADC with response to chemotherapy. Methods Forty patients with advanced colorectal cancer received FOLFOX 4 regimen：oxaliplatin 85mg／m2 iv，d1；leucovorin 200mg／m2 iv，d1，d2；fluorouracil（5-FU） 400mg／m2 bolus，d1，d2；5-FU 600mg／m2，civ 22h，d1，d2. Fourteen days was a cycle and each patient received 12 cycles. The response to chemotherapy was assessed by RECIST 1.1 criteria after 4 cycles treatment. Diffusion-weighted magnetic resonance imaging scan was performed before chemotherapy and at 2nd and 4th cycles post chemotherapy，respectively. Changes of sum of the longest diameter（LD）for all target lesions and tumor ADC values were recorded， and their correlation was analyzed. Results Forty patients were divided into effective group（6 cases of CR，16 cases of PR） and non-effective group（11 cases of SD，7cases of PD）based on the efficacy. In the effective group，ADC values after the 2nd and 4th cycles of chemotherapy were（9.82±2.04）×10-4mm2／s and（10.09±1.95）×10-4mm2／s，significantly higher than（5.63±1.02）×10-4mm2／s before chemotherapy（P=0.009， P=0.001）. While in the non-effective group， ADC values showed no significant difference（P=0.412）.The ADC value before hemotherapy in the effective group was（5.63±1.02）×10-4mm2／s， significantly lower than（8.97±1.88）×10-4mm2／s in the non-effective group（P=0.021）. In the effective group，the ADC value before chemotherapy showed a negative correlation with the decreased sum percentage of the LD for all target lesions after the forth cycle of chemotherapy（r=-0.681，P=0.004）.The change of ADC value of tumors after the second cycle of chemotherapy showed a positive correlation with the decreased sum percentage of the LD for all target lesions after the forth cycle of chemotherapy（r=0.749，P=0.002）.Conclusion The ADC values before chemotherapy and the changes of ADC values in the early stage after chemotherapy can predict the sensitivity of chemotherapy to advanced colorectal cancer， which can be regarded as a sensitive marker of advanced colorectal cancer at the early stage of chemotherapy.