[en] The image quality of proton radiography using the method of beam energy modulation was studied to look into its practical uses. Two different depth-dose distributions generated by modulation were applied to investigate their effects on the density and spatial resolutions of the radiographic image. A steeper slope was found to provide higher resolution for the matching thickness of a phantom. The image was taken on a scintillation screen, and the distance between the phantom and the screen was a sensitive parameter on the resolution. For a beam with a range of 1.2 cm in Lucite, high-resolution images were attainable in the whole range. The resolution of proton images for different kinds of phantoms was examined in comparison with x-ray images as well as images simulated by a Monte Carlo code MCNPX. For a longer range of 18 cm, images attained by simulations indicated that density resolution is better maintained compared to spatial resolution, which is deteriorated by multiple Coulomb scattering