Objectives: The aim of this study was to evaluate the effect of light curing methods in the Knoop hardness number of the resin cement, dual activated or physically activated. Methods: Sixty bovine incisors (6 groups; n=10) had their buccal face ground to expose a flat dentin area. The dentin was treated with the adhesive system Prime&Bond NT (Dentsply), where a mould (1mm in height X 5mm in diameter) was seated. After manipulation (dual or physical mode), the resin cement Enforce (Dentsply) was inserted into the teflon mould and light-cured through a feldspatic ceramic HeraCeram (Kulzer) disks with 8mm in diameter and 1.5 in thickness, simulating the presence of an indirect restoration, using the light curing units: Quartz Tungsten Halogen light-QTH (XL2500, 3M/ESPE)/40s; light emitted by diodes-LED (Ultrablue Is, DMC)/40s and xenon plasma arc-PAC (Apollo 95E, DMD)/3s. The resin cement specimens were flattened to their middle portion and Knoop hardness was measured using HMV 2 microhardness tester (Shimadzu), with a 50g load for 15s, after storage at 37°C for 24 hours. Fifteen measurements were taken for each specimen. Data were submitted to two-way ANOVA and Tukey's test (p=0.05). Results: The Knoop hardness number for dual and physically activated mode were, respectively (Mean±S.D.): QTH–(41.7±5.9) and (34.8±2.7); LED–(50.1±5.2) and (51.2±4.9); PAC–(39.1±4.5) and (0.0±0.0). With dual and physical activation mode, statistically higher values were obtained with LED compared to QTH and PAC. With QTH and PAC, statistically lower values were obtained on physical mode compared to dual mode. With LED, similar values were obtained for both. It was not possible to measure the cement hardness on physical mode when light cured with PAC. Conclusions: The Knoop hardness of the resin cement was influenced by light-curing unit and the cement activation mode.

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