Objective:To study the relationship between plasma DNA levels and metastasis of tumors,evaluate the method of early diagnosis of cancer metastasis. Methods:SPC-A1 were injected into BALB/c mice through the peritoneal cavity. We detected the levels of plasma DNA once a week. Tumors were harvested 7 weeks after tumor cells’ injection,and the metastasis conditions of tumors were also observed. Results:The percentage of mice with plasma DNA increased was 75.6% (34/45),the plasma DNA levels at 3rd weeks and subsequent time points after injection were significantly higher than that at the first week (P < 0.05). Among the mice in group with increased or continued increased plasma DNA,73.5%(25/34) were found lung cancer. However,in the plasma DNA levels non-increased group,27.3%(3/11) were found lung cancer. Statistical analysis showed that plasma DNA levels of continued increased group and increased group was significantly higher than those of non-elevated group (P < 0.05). The levels of plasma DNA correlate with tumor burden(R2 = 0.8703). Conclusion:The levels of plasma DNA increased significantly at an early stage of tumors with the metastasis,the levels of plasma DNA could be used for early detection of tumor metastasis.