To determine the influence of titanium dioxide (TiO 2 ) nanoparticle addition on the opalescence, color, translucency and fluorescence of experimental resin composites.A light curing resin matrix was made by mixing 60wt.% Bis-GMA and 40wt.% TEGDMA. Silane coated glass filler (mean particle size: 1.55μm) was added in the ratio of 50wt.% of the resin composites. A fluorescent whitening agent was also added (0.05wt.%). TiO 2 nanoparticles (<40nm) were added with the concentrations of 0, 0.1, 0.25 and 0.5wt.%. Reflected and transmitted colors of 1 and 2mm thick specimens were measured relative to the illuminant D65 with reflection spectrophotometers. Opalescence parameter (OP), color difference (ΔE* ab ), translucency parameter (TP), fluorescence parameter (FL), and fluorescence and opalescence spectra were calculated.For the 1mm thick specimens measured with 3mm×8mm rectangular aperture, when the concentration of TiO 2 increased from 0% to 0.5%, OP increased from 2.4 to 18.0, TP decreased from 35.4 to 13.1, and fluorescence spectra remained unchanged. Color difference between these specimens was in the range of 3.4–6.6 ΔE* ab units. OP values were significantly influenced by the thickness of the specimens and the configuration of the spectrophotometers (p<0.05).Addition of TiO 2 nanoparticles significantly increased the opalescence of resin composites while leaving the fluorescence spectra unchanged; however, it significantly decreased the translucency and also changed the color (p<0.05). Resin composites with 0.1–0.25% TiO 2 nanoparticle would simulate the opalescence of human enamel.