Objectives: The purpose of this study was to determine color changes in a composite polymerized with various types of curing units after 2 years. Methods: A hybrid (Clearfil AP-X) composite was polymerized with a conventional halogen, a high intensity halogen, a plasma arc, and a light emitting diode unit. Specimens were stored in light proof boxes after the polymerization procedure to avoid further exposure to light and stored in 37°C in 100% humidity (n=5). Colorimetric values of the specimens immediately after curing and after 2 years were measured using colorimeter. The CIE 1976 L*a*b color system was used for the determination of the color difference. Differences from baseline were calculated as E*ab. Data were analyzed with 2-way analysis of variance (p<0.05). Results: The E*ab values varied significantly depending on the curing unit used. The specimens polymerized with plasma arc curing unit induced a significantly higher color changes than any other specimen and the color differences were visually appreciable also for the nonskilled operator (E*ab >2.5). The specimens polymerized with high intensity halogen curing unit produced the lowest color change, however there were no statistically significant difference among the color changes of specimens polymerized with conventional halogen, high intensity halogen and the light emitting diode unit and the color changes were not clinically relevant (E*ab < 2.5). Conclusions: The results of this study suggest that composite materials undergo measurable changes due to the polymerization unit exposure. The specimens polymerized with plasma arc light showed the highest color changes as compared to specimens polymerized with other curing units.