Magnetic nanoparticles (MNPs)/polymer composite nanofibers were prepared via electrospinning of polyacrylic acid (PAA)/polyvinyl alcohol (PVA) aqueous solutions with homogenously dispersed magnetite Fe3O4 nanoparticles (NPs). Experiment results showed that the diameters of Fe3O4 NPs were in a range of 10 nm to 40 nm under all of our synthesis conditions; and Fe3O4 NPs can homogenously disperse in PVA composite nanofibers when they were stabilized by PAA; the crystallinity of PAA/PVA nanofibers increased along with the increase in Fe3O4 NPs content. The magnetic property of Fe3O4 NPs/PAA/PVA composite nanofibers was also investigated and the results showed that the composite nanofibers show a soft ferromagnetic behavior which is the same as Fe3O4 at room temperature. In addition, the water resistance of Fe3O4 NPs/PAA/PVA composite nanofibers was dramatically enhanced by heat treatment. Therefore, in our process to preparing magnetic Fe3O4 NPs/PVA composite nanofibers, PAA acts not only as a dispersant but also as a cross-linking agent.