Objective: To investigate the alteration of circulating DNA level in colorectal cancer patients before and after surgery and its clinical significance. Methods: The circulating DNA levels in 42 patients with colorectal cancer were detected before and on 3, 14 and 30 d after surgery, and the circulating DNA levels in 14 healthy subjects undergoing health maintenance examination during the same period were determined as control. Results: The preoperative circulating DNA level in colorectal cancer patients was significantly higher than that in healthy subjects [(92.25±46.88) ng/mL vs. (22.14±16.16) ng/mL] (P<0.01), and the preoperative circulating DNA level in colorectal cancer patients showed no significant difference between or among the groups stratified by age, sex and tumor location (P=0.293, P=0.244 and P=0.135). In colorectal cancer patients, the circulating DNA level on postoperative day (POD) 3 reached (114.95±62.41) ng/mL, which was significantly higher than that before surgery (P<0.01), but on POD 14 and 30 it was (38.50±37.71) ng/mL and (31.69±41.48) ng/mL respectively, and both were significantly lower than that before surgery (both P<0.01). Conclusion: The circulating DNA level is elevated in colorectal cancer patients, and it is further increased transiently in early postoperative stage and then markedly decreased. It may probably be used as an indicator for postoperative tumor status and efficacy evaluation in these patients.