The　wavelength　selectivity　of　volume　holographic　gratings　with　different　size　ratios　is　presented.　The　gratings　are　made　in　a　doubly　doped　LiNbO3　photorefractive　crystal　by　using　the　transmissive-writing　and　orthogonal-readout　scheme.　The　gratings　are　recorded　at　the　short　wavelength　of　532　nm　and　read　out　at　the　long　communication　wavelength,　which　is　tunable　between　1520　and　1630　nm.　It　is　demonstrated　that　the　wavelength　selectivity　of　gratings　can　be　improved　by　choosing　larger　grating　size　ratios.　A　3　dB　bandwidth　of　0.43　nm　is　achieved　with　the　grating　size　ratio　of　2.5.　The　experimental　results　agree　well　with　the　prediction　of　two-dimensional　coupled-wave　theory.