Monochromators making use of a Fresnel zone plate and a phase zone plate as the dispersing element were fabricated on trial. The monochromators had a pair of pinholes or optical fibers of 0.01 mm diameter for entrance and exit of light. The properties of the monochromators were examined by investigating the shape of spectra of mercury and cadmium discharge lamps in the visible region taking notice of the effects of central stops on the zone plate. Central stops were effective to reduce wings of spectral lines.