Tissue plasminogen activator (tPA), a widely used thrombolytic agent, has an application limit due to short half-life. To prolong the half-life of tPA, liposomes composed of egg phosphatidylcholine (EPC), cholesterol (CHOL) and sodium cholesterol-3-sulfate (CS) were prepared by lipid film method. In addition, distearolyphosphatidyl ethanolamine-N-poly(ethylene glycol) 2000 (DSPE-PEG 2000) was included to give steric barrier to liposomes. Physicochemical characteristics such as particle size, zeta potential, entrapment efficiency and long-term storage stability at 4 degrees C were investigated. The fibrinolytic activity of tPA-loaded in liposomes was confirmed by fibrin clot lysis assay. In vivo pharmacokinetic properties of tPA and the effect of PEG on the blood circulation of tPA-loaded in liposomes in circulation were also evaluated. Both conventional liposomes (EPCL) and PEGylated liposomes (EPC-PEGL) were proper as an injectable formulation with small particle size. Loading process of tPA into liposomes did not alter fibrinolytic activity of intact tPA. Encapsulation of tPA into EPCL and EPC-PEGL prolonged half-life of tPA by 16 and 21 folds compared with free tPA, respectively. Therefore, the use of liposomes could prolong the circulation lifetimes and longevity effect of liposomes on tPA was increased by PEG.