This paper presents an approach to establish the model for predicting the steady-state erosion rate of polyetherimide and its glass fiber composites. Three-factor and two-level, face-centered composite design is used for experimentation. The parameters which affect the erosion rate are selected as glass fiber percentage (0–40%), impingement angle (30 deg–90 deg), and impact velocity (30–90 m/s). Response surface methodology is used to derive second-order quadratic model with interactions. Investigation showed all the parameters have significant effect on controlling steady-state erosion rate of these composites. The interactions of impact velocity-fiber percentage and impact velocity-impingement angle are significant. The increase in erosion rate with the increase in impact velocity is found to be satisfying a power law. Maximum erosion rate for these composites found at around 45 deg–60 deg impingement angle indicates their semiductile erosion behavior. Scanning electron microscopy photographs indicate ploughing, microcutting, development of cracks, and exposure of fibers as the dominating erosion mechanisms for these composites.