BACKGROUND: Chitosan is one of the most significant non-viral vector materials with the advantages of outstanding biocompatibility. Quarternary chitosan derivatives can improve transfection efficiency and solubility of chitosan in a broader range of pH values. OBJECTIVE: To synthesize a new vector of quarternary chitosan and to study its complex conditions with plasmid and transfection efficiency compared with chitosan. DESIGN, TIME AND SETTING: A contrast observational study was performed in Second Affiliated Hospital of School of Medicine of Zhejiang University between August and October 2008. MATERIALS: Quarternary chitosan was synthesized in Polymer Materials Lab of Beijing Institute of Technology. Plasmid pEGFP-C1 was presented friendly by Mr. Zheng of School of Medicine of Zhejiang University. Hela cells were provided by Miss. Cheng of School of Medicine of Zhejiang University. METHODS: Quarternary chitosan was prepared according to mass concentration of 0.2 g/L, pH value 5.5 (or 6.9, 7.6) and sodium acetate concentration of 50 mmol/L, and rapidly mixed with pEGFP-C1. The mixture was swirled for 15-30 second and stood at room temperature for 30 minutes at least. MAIN OUTCOME MEASURES: The impacts of pH values and time on complex ability of quarternary chitosan and plasmid were studied by gel retardation test. Transfection efficiency of quarternary chitosan on Hela cells was observed by inversed fluorescence microscope and also compared with chitosan. RESULTS: Quarternaty chitosan could form complex with plasmid in acidic, neutral and basic conditions. It could be used in a broader range of pH values. In an acidic condition, the combination of quarternary chitosan with plasmid was superior to chitosan. A stable complex was formed via a combination of quarternary chitosan or chitosan with plasmid within 30 minutes, and the stability lasted for 12 hours. Transfection efficiency of quarternary chitosan on Hela cells demonstrated that transfection efficiency of quarternary chitosan was superior to chitosan. CONCLUSION: Quarternary chitosan has a broader range in use and higher transfection efficiency than chitosan; however, there is no significant difference in stability between quarternary chitosan and chitosan. Additionally, transfection efficiency of quarternary chitosan on Hela cells is superior to chitosan, which needs a further research.