Increasing filler content in paper while maintaining paper strength is a continuous need in the paper industry. The bonds between cellulosic fibers and fillers are essential to increase filler level in paper. Besides tensile strength, which traditionally has been used to investigate different fillers on fiber bonding, a new factor, i.e. a filler bondability factor, can be applied in evaluating the mitigation effect of filler addition on fiber bonding. This factor shows its effectiveness in optimizing the key parameters for filler modification and the choice of filler, and it helps to maximize the use of filler in the paper industry.