In　this　paper,　the　phase-type　grating　recorded　in　a　Fe:Cu:LiNbO3　crystal　is　measured　by　dual-wavelength　digital　holography.　In　the　experiment,　a　volume　hologram,　which　is　recorded　in　a　3-mm-thick　Fe:Cu:LiNbO3　crystal　by　interference　of　two　recording　beams　at　the　wavelength　of　532　nm,　is　reconstructed　to　be　imaging　by　dual-wavelength　digital　holography.　Two　lasers　of　the　different　wavelengths　660　nm　and　671　nm　are　used　to　obtain　a　larger　beat　wavelength.　Each　laser　output,　which　is　spatially-filtered　and　collimated,　is　split　into　a　reference　and　object　beams　in　an　interferometer　setup　based　on　the　Mach-Zehnder　configuration.　In　dual-wavelength　phase　unwrapping,　two　individual　phase　images　are　obtained　by　using　each　wavelength,　respectively,　and　the　phase　image　of　beat　wavelength　is　obtained　by　subtracting　one　single　wavelength　phase　image　from　the　other　and　then　adding　2　pi　whenever　the　resultant　value　is　less　than　zero.　In　the　final　synthetic　image,　the　discontinuities　are　removed　after　reduce　the　noise　of　the　beat　wavelength　phase　image.　Thus,　a　3D　surface　profile　of　the　phase　grating　is　obtained.