Introduction: Recently, Bulk-Fill resins have been developed and they can be used with a mono-incremental technique, eliminating the traditional oblique incremental technique and shortening the clinical time. The aim of this study is to evaluate the internal adaptation of composite resins using oblique incremental and monoincremental techniques with sonic activation. Material and method: 15 healthy third molars were used and two enamel finished class II cavity was performed on each one of them. Dimensions were: 3 mm mesio-distal, 2.5 mm buccal-lingual, 2 mm occlusal-pulp depth, and 4 mm occlusal-proximal depth. The distal preparations were restored using a conventional Filtek TM Z350 XT (3M ESPE, USA) resin with oblique incremental technique and the mesial ones were restored using Bulk-Fill SonicFillTM (Kerr, USA) resin through mono-incremental technique and sonic activation. The samples were cut in a mesiodistal direction and analyzed with an optical microscopy at 10× and 40× magnification. The percentage of adaptation of both resins was measured, and a two-way ANOVA with Tukey correction was performed. Results: No statistically significant difference was found in the adaptation of both resins, using oblique incremental and mono-incremental techniques with sonic activation (p>0.01). Discussion: Both techniques reported similar values of adaptation, therefore, the mono-incremental technique with sonic activation is a recently introduced technique, which seems to be less sensitive than oblique incremental technique.