The purpose of this study was to evaluate wear resistance between titanium and cobalt-chromium alloys when opposing to natural teeth; enamel. Surface roughness after polishing and microhardness of metal were also evaluated. Metal specimens were cast from [alpha] titanium (commercially pure titanium; Cp-Ti), and 2 [alpha] + [beta] alloys (Ti-6Al-4V and Ti-6Al-7Nb). Co-Cr alloys were also cast and served as the control. Tooth specimens were selected from extracted premolar for orthodontic purpose. Part I, 4 groups of metal were polished with the same procedure. Roughness average (Ra) of each group was measured by using a surface roughness tester. ANOVA and Bonferroni's multiple comparisons were performed at p < 0.05. Part II, the surface hardness test was performed using a digital microhardness tester (load: 500 g; dwell time: 15 seconds). Statistical analysis was performed as in part I. The correlation between surface roughness and hardness was also assessed. Part III, tooth and metal specimen were secured in and in vitro 2-body were simulator. Wear testing was performed by repeatedly grinding upper tooth specimen on lower metal specimen under flowing water (60 cycles/min, grinding distance: 8 mm. and 100 N dead weight). After 10,000 cycles, wear resistance was assessed by weight loss (WL) of metal specimen and vertical height loss (VHL) of enamel; assessed by profile projector method. MANCOVA and Bonferroni's multiple comparisons were performed at p < 0.05. The surface of Co-Cr alloys was observed to be much smoother than those of Cp-Ti and alloys. The surface hardness of Co-Cr alloys and Ti alloys was significantly higher than that of Cp-Ti (p < 0.05). A significant negative correlation exists between surface hardness and surface roughness with a correlation coefficient of r = -0.71. When opposing to enamel, Co-Cr alloys demonstrated better wear resistance than Cp-Ti and its alloys. Ti alloys exhibited the greatest wear of themselves as well as their corresponded opposing enamel. Moreover, atomic force microscope (AFM) observation after the test revealed that the worn surface of Co-Cr alloys is the smoothest among the examined metals