Cell-free tumor DNA (ctDNA) is a kind of potential tumor biomarkers originated from cancer lesion in the circulating liquids. Liquid biopsy, as a minimally invasive or noninvasive manner, is a cutting-edge technology to detect ctDNA and other circulating biomarkers in the blood or other body fluids. ctDNA is mostly used for cancer patients to select targeted drugs in clinical application. In addition, ctDNA could also be applied to monitor tumor progression and recurrence. In conclusion, ctDNA is a very promising tumor biomarker for diagnosis and monitoring, which would increasingly become a routine clinical application in recent years.