Chitosan (CS) with different viscosity was investigated as a modifier of talc which was used to improve the strength properties and filler retention in high filler content papers. The particle size and morphology of the resulting modified talc were studied to confirm the successful modification. The zeta potential and coating amount of the CS modified talc were also investigated. It was found that CS viscosity was a critical factor for the filler modification and final paper properties. When high viscosity CS was applied, the resulting tensile strength of filled paper was 64% higher than that of unmodified talc at the filler content of 37% as well as maintained favorable optical properties. Furthermore, with the filler loading of 70% (based on the solid fiber weight), compared with that of unmodified talc, filler retention of modified talc was increased by 39%, while the drainage was little influenced. The field emission scanning electron microscopy images showed a better bonding capacity between modified talc and fibers than that of untreated talc. (c) 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013