The micro-mechanical behavior of fiber/matrix interfaces and macroscopic mechanical properties of glass fiber reinforced Nylon 6 composites have been characterized for four different fiber surface treatments using in situ microscopic observation techniques. A pull-out test under an optical microscope is conducted to determine the interfacial bond strength of single-fiber embedded composites. An elasto-plastic axisymmetric finite element analysis is then used to explain the experimentally-obtained different failure mechanisms and processes at a broken fiber end. In addition, micro-deformation and failure process in injection-molded composites are observed under tensile loading in a scanning electron microscope (SEM) for specimens reinforced with randomly-oriented short glass fibers with the same fiber surface treatments. It is revealed that micro-damage and fracture characteristics in injection-molded thermoplastic composites are well correlated with micro-mechanical properties of fiber/matrix interfaces of single fiber composites.