All reinforcing fibers have to be coated with a suitable interlayer (thin film) to ensure their effective functionality in fiber-reinforced polymer composites. The interlayer adhesion on the reinforcing fibers is most critical and decides the performance of the composite. In this study, we demonstrate that interlayer adhesion can be evaluated directly on single glass fibers with a diameter of 19 mu m using the nanoscratch test. The feasibility of the nanoscratch test, commonly used for planar specimens, is examined for glass fibers coated by plasma polymer film with a thickness of 0.10 mu m. The adhesion of thin films on glass substrate can be varied by the deposition conditions. For a given film, the nanoscratch test on fibrous samples provided reliable data resulting in consistent interlayer adhesion for both fibrous and planar glass substrates. (C) 2017 Elsevier B.V. All rights reserved.