Millimeter-wave frequency bands have been considered as candidate bands for the next generation (5G) mobile communications. For successful 5G development, studying millimeter-wave propagation channels is important. Since a line-of-sight (LOS) path between stations is easily blocked by a building in urban environments, diffraction models are important to predict non-line-of-sight (NLOS) wireless channels. However, due to high diffraction loss and environmental constraints, studies on diffraction loss in millimeter-wave bands have not been conducted sufficiently yet. Several investigations of diffraction loss in millimeter-wave bands have been performed, but they are limited by short distance propagation or small diffraction angles. This paper provides a diffraction loss model based on 28 GHz measurements. The distance range of the measurements is up to about 150 m and the diffraction angle range is up to about 50 degrees.