AIM: To prepare thymidine kinase gene (TK gene) nanopartuckes and to investigate the expression of TK gene.METHODS: Poly(D,L-lactic-co-glycolic acid) (PLGA), a biodegradable and biocompatible polymer, was used to prepare recombinant plasmid pEGFP-AFP nanoparticles by a double-emulsion evaporation technique. Characteristics of the nanoparticles were investigated in this study, including morphology, entrapment efficiency, and tissue distribution.The expression of TK gene was also investigated by MTT assay, by which the viable cells were determined after the addition of ganciclovir (GCV). The enhanced green fluorescent protein (EGFP) expression in human hepatocellular carcinoma 5MMC-7721 cells and normal parenchymal Chang liver cells were assessed by flow cytometry.RESUITS: The prepared plasmid-nanoparticles had regular spherical surface and narrow particle size span with a mean diameter of 72±12 nm. The mean entrapment efficiency was 91.25%. A total of 80.14% DNA was found to be localized in the livers after 1-h injection with 32P-DNA-PLGA nanoparticles in mouse caudal vein. The expression of DNA encapsulated in nanopartides was much higher than that in naked DNA, and human hepatocellular carcinoma SMMC7721 cells were more sensitive to GCV than human normal parenchymal Chang liver cells.CONCLUSION: The enhanced transfection efficiency and stronger ability to protect plasmid DNA from being degraded by nucleases are due to nanoparticles encapsulation.