An ultralow reflectivity spiral phase plate (SPP) is proposed to generate an orbital angular momentum (OAM) beam at millimeter-wave frequency. The SPP is composed of unit cells whose equivalent permittivity and thickness are designed to satisfy the impedance matching condition based on transmission line theory. The designed SPP is fabricated by a 3D printing technique. The near-field and far-field phase distributions clearly exhibit the azimuth phase shifting, and the far field directivity patte