In order to improve the safety of lithium-ion secondary battery, a PVdF-HFP/polyethylene (PE) composite gel electrolyte with thermal shutdown function was prepared as an internal safety device. The composite gel electrolyte consists of a PVdF-HFP polymer, PE thermoplastic resin, and 1.0 mol/dm(3) LiClO4/PC + EC (or LiPF6/GBL + EC) plasticizer. When the PE content is over 23 wt.%, the electrical impedance of the composite gel electrolyte can increase rapidly by several orders, around the melting point of PE (mp: 90 or 104-115 degreesC). By the SEM observation it was found that the PE particles uniformly dispersed in the PVdF-HFP gel electrolyte could be fused and formed into a continuous film at or near the PE melting point. The continuous PE film exhibits an ability to cut off the ion diffusion between cathode and anode, thus preventing the cell from thermal runaway. Also, a three-layered film of pure PVdF-HFP gel/ composite gel/pure PVdF-HFP with a higher ionic conductivity and good mechanical strength was prepared for assembling a coin-type lithium-ion cell.