Poly(acrylic acid)-Fe3O4 nanoparticles (PAA-Fe3O4) with a size of 50 nm were synthesized by a one-pot method and then blended with sodium alginate matrix to prepare an ultrathin hybrid active layer of composite membrane for pervaporation dehydration of ethanol solution. The in situ modification of Fe3O4 nanoparticles by PAA improved the nanoparticle dispersion in the membrane. PAA-Fe3O4 nanoparticles not only interfered the ordered packing of polymer chains, but also enhanced the membrane structure stability and the diffusion selectivity. Compared with the membrane blended with Fe3O4 nanoparticles, the membrane blended with PAA-Fe3O4 nanoparticles displayed a much higher separation factor. When the PAA-Fe3O4 nanoparticle content was 8 wt %, the hybrid membrane acquired the optimal pervaporation performance with a separation factor of 1044 and a permeation flux of 1634 g/m(2) h for 90 wt % ethanol aqueous solution at 350 K. Furthermore, the hybrid membrane possessed good long-term operation stability.