Purpose: As polymerization shrinkage is compensated by flow of the composite, several attempts have been performed for relief of the contraction stresses. The aim of this in vitro study was to evaluate the effect of flowable composite or fiber use under composite restorations on microtensile bond strength of composite to dentin in Class I cavities or flat dentin surfaces. Materials and Methods: Twenty-four sound extracted human first or second mandibular molars were randomly assigned to two groups (Class I cavities with a high c-factor or flattened dentin surfaces with a low c-factor). The dentin surfaces were treated with adhesive resin and restored with resin composite using four different techniques (bulk; with flowable composite; with a glass fiber (everStick NET); with a polyethylene fiber [Ribbond]). After 24 h storage at 37°C in water, the specimens were thermocycled 600 times between 5 and 55°C. Microtensile test specimens with a 0.9 x 0.9 (± 0.1) mm2 cross-sectional area were produced, and bond strength tests were carried out at a crosshead speed of 1 mm/min. Mean bond strengths were analyzed using two-way ANOVA and Bonferroni's test at a 95% significance level. Results: Flowable composite decreased dentin bond strength in cavities with a high c-factor (p 0.05). The group restored with everStick NET showed stable bond strengths regardless of the effect of c-factor. Ribbond THM used under composite restorations increased dentin bond strength in cavities with a high c-factor (p 0.05). Conclusion: Flowable composite decreased dentin bond strength in cavities with a high c-factor; however, using a glass fiber in combination with flowable resin, stable bond strengths can be achieved in cavities with a high c-factor. Polyethylene fiber in combination with flowable resin increases the microtensile bond strength to the dentin floor in cavities with a high c-factor. Keywords: c-factor, glass fiber, polyethylene fiber, flowable composites, microtensile bond strengths