Abstract Fiber length and interface properties have significant effects on the mechanical characteristics of fiber-reinforced composites. In this work, effects of screw speed and fiber content on glass fiber (GF) length during the process of twin-screw extruder were investigated. And the influences of GF length on the mechanical characteristics of composites (PET/GF) were also studied. The GF surface was modified by the silane coupling agent (KH-550) with different concentrations of KH-550 solut