Objectives: to determine and compare the microtensile dentin bond strength and the internal adaptation of bulk fill versus incremental fill resin composite restorations applied with two different curing modes in class I high C-factor cavities.Materials and Methods: Forty non-caries human molars were selected for this study. The teeth were divided into two main experimental groups: bulk fill (X-tra fil, Voco GmbH), and incremental fill (Grandio®, Voco GmbH ) resin composite restorative materials. Each group was then divided into two subgroups and cured by LED using either ‘soft start’ or ‘full light intensity’ curing mode. From each group 20 specimens were used for microtensile bond strength assessment, sectioned into small sticks of 1mm x1mm and adjusted to a universal testing machine for bond strength measurement. Another 20 specimens were used for microleakage testing, sectioned mesiodistally after immersion in methylene blue dye and examined under stereomicroscope (x40).Results: A significant difference in microtensile bond strength values of bulk fill and incremental fill resin composite materials when cured in a soft start protocol while no significant difference was found using full intensity curing mode. The results of microleakage scores showed no statistical significant difference between all the tested groups.Conclusions: Within the limitations of this study, Bulk fill resin composite restoration showed comparable microtensile dentin bond strength to those of incremental fill resin composite restoration in high C-factor cavities when using the full intensity curing mode. Bulk fill and incremental fill resin composite restorations exhibited similar internal cavity adaptation when used in high C-factor cavities. The effect of curing modes on bond strength is a material dependent.