Cyclooxygenase (COX)-2 plays critical roles in tumorigenesis, tumor cell growth, and angiogenesis, and inhibiting the expression of COX-2 by gene therapy has showed promising prospects. Vectors are crucial for gene therapy. Polyamidoamine (PAMAM) dendrimers are one type of nano-vectors. In this study, we synthesized a generation 4 polyamidoamine (G4PAMAM) dendrimer/COX-2 antisense oligodeoxynucleotide complex (G4PAMAM/COX-2ASODN), determined the transfection rate of G4PAMAM/COX-2ASODN on cultured breast cancer cells, assessed the cell viability, cell cycle dynamics, and cell invasiveness after transfection, and investigated the effects of G4PAMAM/COX-2ASODN on the expression of COX-2 mRNA and protein and microvessel density (MVD) levels in the tumor tissues of a breast cancer nude mouse model. The results showed that G4PAMAM/COX-2ASODN had a high transfection rate, decreased the cell viability, induced apoptosis and G0/G1 cell cycle arrest, and suppressed cell invasiveness. After treatment with G4PAMAM/COX-2ASODN, the copy number of COX-2 mRNA and protein expression in the tumor tissue were decreased markedly, MVD in the tumor tissue was also decreased, and tumor growth was restrained (p<0. 05). We conclude that COX-2ASODN can be delivered into the cultured and transplanted breast cancer cells efficiently by G4PAMAM, can reduce the expression of COX-2 mRNA and protein, and can lower the MVD of tumor tissues. The G4PAMAM/COX-2ASODN complex has antitumor properties in vitro and in vivo.