Chest radiography is one of the major items for medical diagnostic examination. It is able to diagnose out the fracture of the ribs, clavicles and spine, the diffuse pulmonary infiltration or infection, the pneumothorax, the hemothorax, the pneumonia, …etc. Especially in medical diagnostic therapy, its applications are quite extensive and popular. Nowadays the image recording of chest radiography has been improved from the traditional film to the flat panel detector. Many hospitals get used to operate the radiography processes in the conventional X-ray tube conditions. Neither the relations among the image quality, peak tube voltage and tube current have been systematically explored nor has the effective dose of patient been evaluated. This study uses the medical Revolution XR/d full digital X-ray radiography system, which is the equipment of the radiology department in Hsinchu Macay Hospital and produced by the GE Company, to research. Extremely different from the traditional medical radiography image recorded by the X-ray film, the system equipped with the new developed flat panel detector to record the image in digital method which is able to reserve the image permanently and also to improve the image quality by through the well-developed image processing techniques to aid the radiography diagnosis precisely. Under AEC (Automatic Exposure Control) mode, the experiment adopted the chest quality control phantom to process the chest X-ray radiography under peak tube voltage from 62 kVp to 132 kVp. The calculated SNR (Signal to Noise Ratio) from the lung, heart and subdiaphragm equivalent areas is used as the image quality index. The FOM (Figure of Merit) defined as SNR2/Effective Dose is suggested to evaluate the optimization of dose and image quality. The effective dose is calculated from Monte Carlo Code program MCNP4C combined with the X-ray spectrum produced by the PHOTKOEF program and with the mathematical phantom produced by the BodyBuilder program. This research aims to establish the standard not only for evaluating the optimization of dose-image quality in chest radiography but for comparing the characteristic from the different chest X-ray diagnostic systems. Furthermore, it could be used as a standard to compare the chest X-ray diagnostic systems in different hospitals.