Objective To study the effect of low exposing dose on detecting lung nodules by CT . Methods 26 patients with lung metastasis were selected to scan by CT with four kinds of different protocols , including 195 mAs /6.5mm ( exposing dose/scan thickness ) ,195 mAs/3.2 mm,50 mAs/3.2 mm,30 mAs/3.2 mm in all,then the number,size,position of nodules detected with dfferent dose were calculated respectivly.Results The total number of lung nodules detected with 195 mAs/3.2 mm,50 mAs/3.2 mm and 30 mAs/3.2 mm were no significant difference,but when classificated according to their size,as the exposing dose decreased,the number of nodules under 2 mm or 5 mm was detected decreasingly,morever,the noise of image increased.Although the image-noise of 30 mAs/3.2 mm was significant over 195 mAs/6.5 mm,the number of nodules detected by former was significant more than later.Conclusion As a whole, exposing dose decreased,the noise of image increasesd,the quality of image decreases,the artifact may obscure those small lung nodules, however, exposing dose decreased in a certain extent has no significant influence on CT in detecting lung nodules.Low exposing dose combined with thin slice thickness is more superiority than general dose combined with general slice thickness in sifting lung nodule.