AbstractIn a previous study the authors determined that non-woody materials including brewers’ grain (BG) and oil palm frond (OPF) could be alternatives to wood powder as organic fillers. However, they have the disadvantage of deteriorating the strength of paperboard. If the strength of paperboard could be improved, then one would expect more production cost reductions and bulk improvements by increasing the addition of organic fillers. In this study, surface modification of organic fillers was used as a method to improve paperboard strength. The goal was to find the most effective condition for surface modifications. Surface modifications of BG and OPF fillers were carried out using cationic and oxidized starches, and the strengths and reductions in the drying energies of the sheets were measured. The zeta potentials of the modified organic fillers showed that the surface modifications were performed properly. Surface modification with starches improved the bulk and strength of the sheets simultaneously, and modification with the addition of a large amount of cationic starch was more effective in improving the strengths and the reductions in drying energies of the sheets than using cationic and oxidized starches together.