This paper presents an experimental investigation into the effects of water temperature and immersion times on the tensile strength degradation of plain-woven glass fiber reinforced plastics (GFRP) laminates. GFRP specimens were tested as-received and after hydrothermal aging in deionized water at 40 °C, 80 °C, 95 °C to evaluate their tensile properties. The strength and rupture strain had a tendency to decrease drastically in the early stages and to saturate toward certain strength with long-term aging, regardless of the water temperature. The fracture surfaces were examined by scanning electron microscopy (SEM) to study the fracture mechanisms of woven GFRP after hydrothermal aging. While the strength of the glass fiber decreased, the fracture surfaces of the E-glass fibers flattened, and the mirror zones on the fracture surfaces enlarged. Interfacial degradation was confirmed by fiber pull-out, and the debonded fibers showed no resin matrix adhesion to the fiber surfaces. These experimental results suggest that the degradation in the strength of woven GFRP is dominated by degradation of their fiber reinforcement and the fiber/matrix interface.