In　order　to　investigate　the　wavelength　selectivity　of　volume　holographic　gratings,　applied　as　wavelength　division　multiplexing　(WDM)　filter,　a　532　nm　laser　was　used　to　record　volume　holographic　gratings　with　finite　size　in　doubly　doped　cubic　lithium　niobate　crystal.　Gratings　of　different　size　ratio　were　recorded　in　transmission　configuration　one　by　one　and　read　in　orthogonal　scheme　with　a　tunable　infrared　laser　working　in　the　optical　communication　band.　The　3　dB　bandwidth　of　wavelength　selectivity　curves　corresponding　to　the　grating　size　ratio　of　4:3,　4:2　and　5:　2　were　0.48　nm,　0.45　nm　and　0.43　nm　respectively.　The　Bragg　central　wavelength　of　each　grating　is　almost　the　same　as　theoretical　prediction.　This　indicated　that　with　larger　gratings　size　ratio　one　can　obtain　narrower　3　dB　bandwidth　in　the　readout　of　volume　holographic　gratings.　The　wavelength　selectivity　of　volume　holographic　WDM　filter　can　be　obviously　improved　by　choosing　a　suitable　grating　size　ratio　and　it　is　possible　to　meet　the　requirement　of　dense　WDM.