In this work, we have synthesized a polycation and a polyanion via a combination of oxyanion-initiated polymerization and polymer reaction, and then developed a novel approach to prepare a controlled magnetic target gene carrier with magnetic Fe(3)O(4) nanoparticles as core and poly(ethylene glycol) (PEG) segment as corona via layer-by-layer (LbL) assembly and shell-cross-linking. Magnetic nanoparticles (MNPs) were first modified by poly[2-(dimethylamino)ethyl methacrylate] (PDMAEMA) via radical polymerization. The resulting MNPs were used to compact deoxyribonucleic acid (DNA) through LbL assembly, involving four steps: ((1)) the binding of DNA to the polycation PDMAEMA on the surface of MNPs; ((2)) the produced particles in Step 1 with negative charge interacting with additional polycation ethoxy group end-capped PDMAEMA (EtO-PDMAEMA) homopolymer, leading to a positive charge surface; ((3)) using carboxyl group (-COO(-)) of poly(methacrylic acid) (PMAA) in a diblock copolymer (MePEG2000-b-PMAA(SH)) as polyanion, which has partial mercapto groups (-SH) in PMAA segment, to interact with the particles produced in Step 2; ((4)) the shell of the composite nanoparticle was crosslinked by oxidizing the -SH groups of the MePEG2000-b-PMAA(SH) to form disulfide linkage (SAS). All the processes of LbL assembly were investigated by agarose gel retardation assay and zeta potential measurements. The in vitro cytotoxicity analysis proves that polyions/DNA MNPs have excellent properties and potential applications as gene carriers. (C) 2011 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 49: 4081-4091, 2011