This study aimed to establish an evaluation standard for digital chest X-ray radiography. The quality control phantom model 07-646 produced by Nuclear Associates Co. was used as a standard phantom. Chest images were acquired from two different digital chest X-ray radiographic systems. SNR (Signal to Noise Ratio) was extracted from the images to evaluate the image quality. FOM (Figure of Merit) defined as the square of SNR divided by the effective dose was applied as an index for dose-image quality optimization.