The purpose of this study was to evaluate the color changes of composite resin polymerized with three type of light curing units. Composite resin (Z100, shade A2) were applied in a cylindrical metal mold(2 mm thick, 7 mm diameter). Twenty specimens according to light curing units were made. Group 1 : the specimens were polymerized with Apollo 95E for 3seconds(1370 mW/$ extrm{cm}^2$). Group 2 : the specimens were polymerized with XL 3000 for 40seconds (480 mW/$ extrm{cm}^2$). Group 3 : the specimens were polymerized with Spectrum 800 for 10 seconds(250 mW/$ extrm{cm}^2$) and 30 seconds(700 mW/$ extrm{cm}^2$). The microhardness values(VHN) of upper and lower surfaces specimens after light polymerization were measured for the degree of polymerization. All specimens were stored in distilled water at 6$0^{circ}C$ for 30 days. The color characteristics(L$^*$, a$^*$, b$^*$) of the specimens before and after immersion were measured by spectrophotometer and the total color difference ($Delta$E$^*$) was computed. The results obtained were as follows : 1. The microhardness values of Group I showed significantly lower than those of Group II and III (p<0.05). 2. In all groups the $Delta$E$^*$ values presented below 2.0. 3 Group I showed the highest $Delta$E$^*$ values followed order from highest to lowest by Group II and III (p<0.05).