The current study investigated the effect of different ceramic thicknesses and substrates on color parameter of Yttria-zirconia ceramics spectrophotometrically.In the present study, twenty all ceramic disc specimens 10 mm in diameter divided into two groups according to the different thicknesses 0.5mm and 0.8mm (10 discs for each), were milled from yttria stabilized zirconia polycrystal (Y-TZP) blocks using CEREC inlab system. The samples from each group were subdivided into two subgroups according to the substrate. Twenty Discs made of resin composite and base metal alloys (10 discs for each) were used as substrate. Finally the ceramic discs were luted to the substrate discs using dual cure resin cement. A special alignment device was constructed to insure a constant 3 kg load to cementation samples in uniform thickness and without eccentric movements. Spectrophotometer was used to measure color difference(ΔE), lightness (L*), chromaticity (a*) and (b*) for 0.5 and 0.8 Y-TZP discs before cementation as control group and after cementation to substrate.Within the limitation of the present in vitro study the following conclusions can be drawn:1-The color parameter of yttria stabilized zirconia polycrystal (Y-TZP) when used as core material, was dependent on its thickness and the type of substrate material used underneath.2- Color difference was dramatically increased when the thickness of yttria stabilized zirconia polycrystal (Y-TZP) decreased from 0.8mm to 0.5mm, when using base metal substrate material.3- Yttria stabilized zirconia polycrystal (Y-TZP) restoration had the ability to mask the substrate was not affected by the substrate color, when the ceramic thickness is 0.8mm or more.