X-ray beams irradiated from an x-ray tube with a point source or a sufficiently small focal spot produce edge-enhanced images in the boundary of an object due to the effect of phase shift of x-rays. This technique is called phase contrast imaging. A digital phase contrast imaging system based on a photostimulable phosphor plate (imaging plate) designed for mammography has been developed for clinical use recently and now commercially available. In this study, the digital phase contrast images of an acrylic plate and plant seeds were acquired without any increase of incident dose to detector when compared to conventional contact digital imaging. Improvement of image edge sharpness was evaluated in terms of spatial edge response and spectral analysis of the images. In addition, the improvement of the sharpness of the image was also evaluated in clinical mammograms. Our results indicated that higher image sharpness in the boundary of the object was observed. The power spectrum of the digital phase contrast image was found to be higher than that of the digital contact image at wide spatial frequency region. In conclusion, the commercially available phase contrast imaging system can provide breast images with details that are not available in conventional mammograms. The digital phase contrast imaging would be useful to detect diseases, especially microcalcifications, in mammograms without any increase of exposure dose.