We evaluated the image quality and the effects of scatter decreasing using various magnification factors, so as to investigate the usefulness of a semi-magnification technique for mammographic. The measured scatter fraction indicated that anti-scatter efficiency of this semi-magnification technique was inferior to that of using a grid. An observer performance test, using RMI-156 breast phantom images, was carried out using 7 radiological technologists. The results indicated that this semi-magnification technique (factor 1.2 and 1.3) with a grid was significantly better to the non-magnification technique (factor 1.0). This was probably bacause the effective noise was reduced and the detectabilities for lesions were improved due to radiographic magnification. Considering the image quality and the object area that can be projected on a film, the technique using magnification factor of 1.2 with a grid was the most suitable for screening mammographic screening.