AbstractThe effect of the ultrasonic/mechanical stirring method (physical treatment) and 3‐glycidoxypropyltrimethoxy silane (GPS, chemical treatment) modifications on the sugarcane fiber (SGF)/epoxy composites was investigated to improve the interfacial interaction between fiber and epoxy resin. An optimum concentration GPS at 2 wt% was found for the best improvement in tensile strength at 40 wt% SGF content based on the evaluation of GPS content, indicating the role of GPS as an effective coupling agent. Overall, the best improvement in tensile strength was observed for GPS‐modified cases, and particularly tensile strength reached up to 32.9 MPa at 50 wt% of fiber content. In addition, three types of Polyurethane (PU)‐modified epoxy with different polyols at various SGFs contents were investigated. In all cases, the tensile strength of SGF/PU‐epoxy cases was higher than that of SGF/epoxy cases due to the relatively lower viscosity of PU‐epoxy to easily wet the SGF surface. In general, impact strength for samples without physical or chemical treatment also ranked the lowest value in most cases. The enhanced interfacial interaction between SGF and epoxy for surface‐modified cases was suggested through the morphology observation, dynamical mechanical analysis, and the onset degradation improvement in thermal stability.