The pattern size in current and future photomask system will be very fine. The high uniformity of optical properties, critical dimensions, errors are required. The layer thickness estimation is one of most important subject for the design up of high quality photomask blank. To control photomask specifications, layer thickness evaluation is a key technology. High precision and accuracy measurement method has been needed. The grazing-incidence X-ray reflectivity method is very useful in order to measure thickness, density and interface roughness of layers in photomask blank system. It is a nondestructive and an absolute measurement method. In this paper, we discuss the correlations between measured optical density, reflectivity distribution of photomask blank and layer thicknesses which were calculated X-ray reflectivity method. A Cr oxide / Cr nitride thin film photomask blanks were prepared with DC sputtering method. The X-ray reflectivities of those photomask blanks were measured with RIGAKU ATX-E diffractometer system with asymmetric channelcut monochrometer. The thicknesses maps of the photomask blanks were calculated with RIGAKU XDD° program system. The optical properties distributions were measured with spectorophotometer and optical density meter. The correlations between the optical properties and layer thicknesses, chemical compositions were evaluated. The current photomask blank has high uniformities of chemical compositions, surface geometries, crystal structures and other properties. We evaluated the correlations between photomask properties and layer thicknesses. The optical properties distributions are affected by layer thicknesses distribution of photomask blank.