Objective:To investigate the feasibility of gene transfection of Hela and A549 cells mediated by cationic polymer-polyethylenimine(PEI) and evaluate the effect of cell cycle status on efficiency of PEI-mediated gene delivery. Methods: Hela and A549 cells were synchronized into different phases of the cell cycle by 6 MV X ray radiation or drugs or serum deprivation. The synchronized cells were transfected using PEI carrier at various PEI/DNA ratio(P/D=2/2,4/2,6/2,8/2,10/2-滋g/-滋g),and the transient transfection efficiency of enhanced green fluorescence protein (EGFP) gene in these cells was determined by flow cytometry and fluorescent microscopy. The relationship between cell cycle status and the efficiency of transgene expression was analyzed. Results:Hela cell could be synchronized in S phase by X ray radiation or incubation with Cisplatin(PDD),and A549 cell could be synchronized in G2/M phase by incubation with Docetaxel(TXT) at certain dose-time condition. Both Hela and A549 cells were synchronized in G0/G1 phase serum deprivation. Transfection efficiency in synchronized cells was different at different phases of the cell cycle. PEI showed the minimal transfection efficiency at G0/G1 phase and the medium at G2/M phase and the maximum at S phase(P < 0.05,S,G2/M phase group vs control; P < 0.0001,S,G2/M phase group vs G0/G1 phase). Conclusion:PEI is an effective and safe carrier for gene delivery. Cell cycle status restricted the expression efficiency of transfection mediated by PEI. Transfection using PEI is influenced by cellular uptake,transportation into nucleus and transcription,and those processes are cell-cycle-dependent.