Signal transducer and activator of transcription 3(STAT3) plays an important role in tumor occurrence and development. Here we attempt to evaluate the clinical diagnostic value of STAT3 as a potential tumor marker by detecting the expression of STAT3 in blood of non-small cell lung cancer (NSCLC) patients. Immunohistochemistry was applied to detect STAT3 protein in A549 cells and MRC-5 cell, real time PCR was applied to detect STAT3 mRNA and ELISA was applied to detect the expression of STAT3 protein in NSCLC patients, benign lung disease group and healthy controls. The expression levels of STAT3 mRNA and STAT3 protein in A549 cells and NSCLC patients were significantly higher (P < 0.01). The areas under ROC curves (Az) of STAT3 mRNA and STAT3 protein were 0.870 and 0.860 respectively, which had no significant difference (P > 0.05). When comparing the Az of STAT3 protein with CEA, CA125 and CYFRA21-1 respectively, there was no significant difference (P > 0.05). And we compared the Az of the four joint detection (STAT3 + CEA + CA125 + CYFRA21-1) with the three joint detection (CEA + CA125 + CYFRA21-1), 0.930 and 0.841, respectively, P < 0.05. It appears that the diagnostic value and accuracy of STAT3 is equivalent with CEA, CA125, and CYFRA21-1 in NSCLC, and to joint with other three can achieve higher diagnosis value.