The computed tomography is an useful technique of detecting and following the malignant tumor of chest, but the high dose examination actually can not be neglected. Due to the lung is filled by air. The x-ray penetration attenuation is lower as compared with other soft tissue in the thorax. The aim of the reserch emphatically lies in discovering that whether the low radiation dose computed tomography can maintain the diagnosis quality. In the study, all of the patients have malignant tuomr of lung and follow up computed tomography regularly. After comparision with the low dosage radiation computer tomography scanning, and the primitive dosage computed tomography scanning. Appraisement whether such low dosage computed tomography scanning image can affect its diagnosis value is made. At the same time, the facts which may be able to affect the image quality, including patient's sex, age, body weight and BMI are disscused in the study.