Purpose: This study was carried out to investigate the microhardness of composite resin polymerized with three different light sources. Materials and Methods: Thirty cylindrical samples (6 x 2 mm) were divided into three groups as follows: G1, conventional light polymerization for 40 s at 600 mW/cm²; G2, soft-start polymerization for 40 s varying from 100 to 450 mW/cm²; G3, argon laser irradiation for 10 s at 250 mW/cm². All samples consisted of a single layer of composite (Z100, 3M) and were stored in distilled water at 37°C ± 2°C for 24 h. Vickers microhardness tests were then conducted under 50gf load for 15 s on the upper and lower surfaces of the samples. Data were submitted to two-way ANOVA and Tukey’s test (5%). Results: ANOVA demonstrated significant differences of the main factors (light source and surface evaluated). The Tukey’s test confirmed different treatment results (conventional: 115.50 ± 14.44 HVN; soft-start: 99.88 ± 10.63 HVN; laser: 70.11 ± 15.60 HVN) and surfaces evaluated: upper surface (conventional: 123.78 ± 9.69 HVN; soft-start: 109.38 ± 3.66 HVN; laser: 80.59 ± 12.10 HVN) / lower surface (conventional: 107.23 ± 13.93 HVN; soft-start: 90.40 ± 5.01 HVN; laser: 59.63 ± 11.10 HVN). Conclusion: Laser irradiation resulted in lower microhardness values on both upper and lower surfaces compared to conventional light curing and soft-start polymerization with halogen light, while conventional and soft-start polymerization presented similar results.