In this study, an antisense c-Ha-ras DNA was synthesized. It was a complementary oligonucleotide chain to the sequence of initiation position of c-Ha-ras gene transcription. We found that the antisense DNA blocked the gene expression at the level of transcription and translation and partially reverted the malignant phenotyped of gastric transformed cells, including their growth rate, colonies formation in soft agar, tumorigenicity in nude mice and different grade. When the 5'-end of the antisense DNA was covalently linked with psoralen group, its biological effects were significantly enhanced. No effect of tumor inhibition was found in control oligonucleotide chains not complete complementary to the sequence of target gene. These results indicate that antisense DNA and its derivative are effective in tumor gene therapy.