Aim: To compare microleakage in premolar class I restorations between nanohybrid and microhybrid composites in an in vitro study. Materials and Methods: Overall, 32 healthy premolar permanent teeth extracted for reasons unrelated to the study were used. Class I Black restorations were performed and divided into two groups. Group A: microhybrid resin, Group B: nanohybrid resin. Both groups were subjected to manual thermocycling (300 cycles at 5, 37, and 55°C), and they were then immersed in 2% methylene blue during 24h. Subsequently, the samples were washed, dried, sectioned, and observed under a stereoscopic microscope. Results: In the microhybrid resin composite (MRC) group, only two teeth (28.8%) did not show filtration (Grade 0), whereas eight of the specimens evaluated (80%) showed Grade 3 filtration (dye penetration to the pulpal floor). On the other hand, in the nanohybrid resin composite (NRC) group, the highest prevalence was found in Grade 1 (no dye penetration) in eight specimens (66.7%). There was no statistically significant association between the degree of filtration and the type of resin composite used (P = 0.089). Conclusions: Both materials showed microleakage, but the microhybrid resin presented a higher degree of filtration compared with the nanohybrid resin. No statistically significant association was found with the degree of microleakage between the resins.