Marginal microleakage is one of the major disadvantages of resin composite restorations. The purpose of this study was to evaluate in vitro the effect of reinforcement with fiberglass in Class I restorations with composite resin on the marginal microleakage.The sample was composed by 30 human third molars, which were divided into 3 groups: G1 (no fiberglass – control group), G2 (1 layer of fiberglass) and G3 (2 layers of fiberglass). The restorations were subjected to a total of 300 thermal cycles and were immersed in solution of methylene blue 2%. The analysis of microleakage was performed in a stereomicroscope and measured Data were analyzed statistically by the Chi-square and ANOVA tests at 5% significance level. G3 presented the lower infiltration index, but it was not statistically significant (p>0.05), indicating that use of fiberglass did not appears effective against microleakage. Although the use of layers of fiberglass slightly reduces polymerization shrinkage, the incremental technique seems to be more viable clinically.