The stability of poly(vinyl alcohol) (PVA) nanofibrous mats in water media was improved by post-electrospinning treatments. Bifunctional glutaraldehyde (GA) in methanol was used as a crosslinking agent to stabilize PVA nanofiber, but fiber twinning was observed frequently, and the highly porous structure of PVA nanofibrous mats was destroyed when the crosslinked fiber was soaked in water. To overcome this shortcoming, chitosan (CS) was introduced into the PVA spinning solution to prepare PVA/CS composite nanofibers. Their treatment in GA/methanol solution could retain the fiber morphology of PVA/CS nanofibers and porous structure of PVA/CS nanofibrous mats even if they were soaked in aqueous solutions for 1 month. Scanning electron microscopy (SEM), X-ray diffraction (XRD), thermal gravimetric analysis (TGA) and differential scanning calorimetry (DSC) were applied to characterize the physicochemical structure and thermal properties of PVA nanofibers. It was found that the water resistance of PVA nanofibrous mats was enhanced because of the improvement of the degree of crosslinking and crystallinity in the electrospun PVA fibers after soaking in GA/methanol solution.