The emulsions with various oils such as linseed oil, soybean oil and squalene were prepared to obtain the relationship between the stability and the transfection activity of the emulsions. 1,2-Dioleoyl-sn-glycero-3-trimethylammonium-propane (DOTAP) was used as a single cationic lipid emulsifier. The droplet sizes and size distributions of DOTAP emulsions were dependent on oils which had different interfacial tensions. The droplet sizes followed the order of squalene emulsion<soybean oil emulsion<linseed oil emulsion. The squalene emulsion was the most stable carrier since it kept its integrity in serum and PBS solution. For in vitro gene transfer, the transfection activities of the lipid carriers in the presence of serum followed the order of squalene emulsion>soybean oil emulsion>linseed oil emulsion>DOTAP liposome. The squalene emulsion showed the least cytotoxicity with or without serum. For in vivo gene transfer, the squalene emulsion also had the most potent transfection activity in the mouse after intravenous administration. Squalene as the oil component can enhance the stability of cationic emulsion more effectively that could be useful for the transfer of genes in vitro and in vivo.