Abstract： Objective To investigate the mobility of the lung tumors in order to guide the margin that CTV is to be expanded to PTV during stereotaetic radiotherapy for lung tumors.Methods Eighty-three lung tumom ineluded,in which 7 invaded the chest wall.1-2 gold marker(s)was/were implanted into or nearby the tumor.The motion of lung tumors in x(right-left),y(superior-inferior)and:(anterior-posterior)directions were measured via X-ray simulator system.One week later,1.25 mm thick of 16-slice spiral CT lung scan were taken.Apex,middle,and bottom of the lung were classified according to anatomic boderlines.Results No significant difference was detected among movements of apex,middle and bottom of the lung lesions in x axis direction((1.6±0.8),(2.6±1.1),(2.8±1.7)mm,respectively(F=1.76,P=0.179))and,y axis direction((2.0±1.3),(4.3±2.8),(4.0±2.0)mm,respectively(F=2.06,P=0.135))in 76 lesions of 63 patients.Significant differences were detected in:axis((2.7±0.8),(7.3±3.6),(12.0±5.3)mm(F=16.33,P=0.000)).The mobilities of the 7 lesions of 7 patients invading the chest wall were very small in all directions(0.9±0.7,0.7±0.7.and 2.0±0.6 mm,respectively).Motion amplitude did not correlate with the sex,age,height,weight and FEV1/FVC(r=-0.001-0.136.all P>0.05).Conclusions Motion amplitude of lung tumor due to breathing is associated with tumor loeation and status of chest wall invoivement.Identification of the mobile rules and the influencing factors is important for the internal margin set-up in lung tumor radiotherapy.