Long-term weathering tests revealed that glass fiber-reinforced cement (GFRC) might exhibit tensile strength reduction and ductility loss with aging. The main goal of this research was to assess the effectiveness of polyvinyl alcohol (PVA) powder in enhancing the durability characteristics of GFRC sheets. Accelerated aging was achieved by using low pressure steam curing. The strength and ductility of GFRC sheets were measured by the direct tension test. The test results show that the incorporation of PVA powder into GFRC improves the mechanical behavior and changes the failure mode from brittle to ductile. To investigate the mechanism of the enhancement, the fiber-matrix interface was examined by polarizing optical microscopy and scanning electron microscopy (SEM) with energy dispersive X-ray analysis (EDAX). It was found that the addition of the PVA powder results in the deposition of a polymeric film on the fiber surface and thus prevented the accumulation of calcium hydroxide in the interfacial zone. The use of the PVA powder led to a more ductile interfacial microstructure and to better bonding between fiber and matrix, which is believed to be responsible for the tensile property enhancement.