Abstract: Objective To investigate the expression of plasma miR-21 in patients with colon carcinoma before and after operation and its clinical significance. Methods The plasma miR-21 expression and its relationship with clinical pathological parameters were measured by quantitative real-time PCR(qRT-PCR). In addition, the expression levels of plasma miR-21 in colon cancer patients were measured one week before operation, one week and one month after the operation. Results The plasma miR-21 expression in 40 colon cancer patients before operation had significant difference compared with control group(P<0.05), related to the degree of tumor differentiation, clinical stage, lymph node metastasis and distant metastasis(P<0.05), but age or sex(P>0.05). The plasma miR-21 expression one week and one month after the operation were significantly decreased, compared with that before operation(P<0.05). Conclusion The plasma miR-21 expression may be used as a potential biomarker for colon cancer, to be a new effective auxiliary method for the diagnosis and prognosis of colon cancer patients.