The objectives of this study was to evaluate current visible light curing units regarding microhardness and microleakage. Fourty samples of composite resin(Z-250, 3M) were cured by different light curing units(Flipo, LOKKI; Credi II, 3M; XL 3000, 3M; Optilux 500,Demetron) in acrylic blocks. Microhardness was measured using a calibrated Vickers indenter on both top and bottom surfaces after 24 hours of storage in air at room temperature. Class V cavities were prepared on buccal and lingual surfaces of fourty extracted human molars. Each margin was on enamel and dentin/cementum. Composite resin(Z-250, 3M) was filled in cavities and cured by four different light curing units(Flipo, LOKKI; Credi II, 3M; XL 3000, 3M; Optilux 500, Demetron).

The results of this syudy were as follows:

Microhardness

1. Flipo showed low microhardness compared to Optilux 500, Credi II significantly in upper surface. Flipo didn't show a significant difference compared to XL 3000.

2. The microhardness resulting from curing with Flipo was lower than that of others on lower surfaces.

Microleakage

1. Dentin margin showed significantly high dye penetration rate than enamel margin in all groups(p<0.05).

2. No significant differences were found on both enamel and dentin margin regarding curing units.