Abstract: Objective　To compare the color differences of different metal bases and different opaque thickness, and evaluate the best thickness of opaque on differentmetal bases.Methods　Preciousmetal, gold sediment and Ni-Cr plateswere prepared as bases, then opaque samples of 0.05, 0.1, 0.2, 0.3, 0.4 and 0.5 mm thickness were fabricated on such plates. Minolta chro- matics meter CR-321 was used to examine the color properties of these samples, andΔEwas calculated to evaluate the color differ- ence of the specimens.Results　For the restoration based on Ni-Cr and precious alloys, the color of restorations was not affected by the color of metal bases as the thickness of opaque reached 0.3 mm, and the chromatic valueΔE<1.5 NBS. For the restora- tion based on gold sediment, the color of restoration was not affected by the color of metal as the thickness of opaque reached 0·1 mm, and the chromatic valueΔE<1.5 NBS.Conclusion　Different opaque thicknesswas necessaryto obtain ideal color ap- pearance in clinic. As the opaque thickness increased, the color of restoration based on Ni-Cr and noble metal increased but the color of restoration based on gold sediment decreased.

Key words: metal base, opaque thickness, color, porcelain-fused-to-metal alloy