We report a facile method to immobilizing silver nanoparticles (AgNPs) into electrospun polymer nanofibers. In this approach, water-stable polyacrylic acid (PAA)/polyvinyl alcohol (PVA) nanofibers, fabricated by electrospinning a 10 wt% PAA/PVA solution and subsequent heating treatment at 145 degrees C for 30 min, were used as nanoreactors to complex Ag(I) ions through binding with carboxyl groups of PAA for the subsequent reductive formation of AgNPs. The as-prepared AgNP-immobilized nanofibers are thoroughly characterized by scanning electronic microscopy, transmission electron microscopy, energy dispersive spectroscopy, selected area electron diffraction, Fourier transform infrared spectroscopy, and thermogravimetric analysis. Moreover, the effect of AgNO(3) solution concentration on the morphology of hybrid nanofibers, Ag content, the size of AgNPs, and catalytic activity of hybrid nanofibrous mats are systematically investigated. We show that spherical AgNPs are uniformly distributed along the cross-section of nanofibers. X-Ray diffraction indicates that the formed AgNPs in the nanofibers are crystalline. The AgNP-immobilized nanofibrous mats exhibit superior catalytic reduction capacity to 4-nitrophenol with efficiency approaching 100% within 30 min and excellent reusability. Furthermore, the size and spacial distribution of AgNPs can be tuned by varying the AgNO(3) solution concentration, thus manipulating the catalytic activity of AgNP-immobilized nanofibrous mats. The strategy to immobilizing and manipulating the size of the AgNPs within polymer nanofibers may be extended to other particle systems for various applications in catalysis, energy, sensing, photonic and biomedical applications.