Cardiac enlargement is an important symptom of vascular and heart disease. The cardiothoracic ratio (CTR) is an important index used to measure the size of heart. The aim of this study was to assess the relationship between aging and cardiothoracic ratio. This paper also presents an improved C-V level set method to segment lung tissue based on X-ray image, which used to automatically compute CTR. In the investigation carried out in our school, we got more than 3 120 chest radiographs from medical examination of the working population in Beijing, and we systematically studied the effects of age and gender on the CTR to obtain reference values for each group. The reference values established in this study can be useful for recording and quantifying the cardiac enlargement, so that it may be useful for calling attention to the cardiovascular diseases and the heart diseases.