objective To investigate the toxicity and transfection efficiency of magnetic chitosan nanoparticle as a vestor mediated eNOS gene delivery to vascular smooth muscle cells in vitro,and then the gene expression and effect on ballon-injured VSMC proliferation were also detected. Methods Prepare chitosan coated magnetic nanoparticles by the in-situ co-precipitation reaction and couple eNOS plasmid.The cytotoxicity to VSMC of magnetic chitosan was evaluated by MTT．In the apposite magnetic field,recombinant adenovirus carried eNOS (AdCMV-eNOS) ,magnetic chitosan-eNOS complex and magnetic chitosan alone were transfected respectively into the cultured ballon-injured VSMCs.The expression of the eNOS gene was detected by immunofluorescence and RT-PCR,and the VSMCs generation cycle and apoptosis were examined by flow cytometer.Results Below the concentration of 20mg/ml, magnetic chitosan nanoparticle has little toxic effect on VSMC proliferation. Though the transfection efficiency in magnetic chitosan-eNOS complex group（58.7±3.6%）was lower than that of AdCMV-eNOS group（78.1±2.5%）,the expressions of eNOS gene in VSMCs were detected in both groups by immunofluorescence and RT-PCR. By flow cytometer VSMCs in G0/G1 stage were detected more in both groups above-mentioned than the magnetic chitosan empty vestor group after gene transfer,while cells in S/G2-M were less.And no significant differences were found in apotosis among the three groups. Conclusion Normal concentration of magnetic chitosan nanoparticle vector has no inhibition on VSMC proliferation. It can efficiently mediate gene transfection into VSMCs in vivo and delays the growth cycle of VSMC.