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.

Group1: the specimens were polymerized with Apollo 95E for 3seconds(1370 mW/cm²).

Group2: the specimens were polymerized with XL 3000 for 40seconds(480 mW/cm²).

Group3: the specimens were polymerized with Spectrum 800 for 10 seconds(250 mW/cm²) and 30 seconds(700 mW/cm²).

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 60℃ 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 (ΔE^*) 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 ΔE^* values presented below 2.0.

3. Group I showed the highest ΔE^* values followed order from highest to lowest by Group II and III (p<0.05).