Objective: To investigate the effect of filler content on microtensile bond strength (µ-TBS) in Class I cavities. Method and Materials: Experimental composites with filler contents of 80%, 76%, 70%, 60%, and 50% by weight were used. Polymerization shrinkage was measured with Acuvol, and a three-point flexural test was performed to determine flexural properties. For evaluation of µ-TBS, 25 extracted human molars were randomly divided into five groups and Class I cavities were prepared. After filling with one of the experimental composites and curing for 40 seconds, teeth were serially sectioned perpendicular to the cavity floor. Stick-shaped samples were tested with a microtensile tester. Statistical analysis was conducted using ANOVA and Pearson correlation tests. Results: Significant correlations were found between filler content and polymerization shrinkage (r = -0.973, P .05) and the filler content and Young modulus (r = 0.891, P .05). Different filler contents in the experimental composites had no significant effect on µ-TBS or flexural strength. Conclusion: In a Class I cavity model, this in vitro study showed that the filler content did not influence the flexural strength of experimental composite resins and had no effect on the microtensile bond strength between composite resin and dentin. Keywords: composite resins, filler content, flexural modulus, flowable composites, microtensile bond strength, polymerization shrinkage