Objective:To observe the effect of Porphyromonas gingivalis(Pg) on the production of nitric oxide(NO) in cultured human umbilical vein endothelial cells(HUVEC), and investigate the pathway of Pg in damaging in endothelial function. Methods: HUVEC were treated with PgATCC33277 at multiplicity of nfection(MOI)1:10,1:100 for 4,8,12,24 h respectively, nitric oxide production was assayed by measuring the accumulation of nitrite in culture supernatants; western-blot analysis of inducible nitric oxide synthase(iNOS) and endothelial nitric oxide synthase(eNOS) was carried out; HUVEC co-coltured with Pg were observed by transmission electron microsocopy(TEM). Results:It was found that Pg can induce iNOS expression, inhibit eNOS expression and stimulate release of nitric oxide. Under TEM, it was found that Pg appeared to adhere HUVEC by fimbria and invade into HUVEC. Conclusion:The present study clearly showed that Pg adhered,invaded and existed in HUVEC, Pg induced iNOS expression, inhibited eNOS expression and stimulated release of NO. The ability of Pg to promote the production of NO may be important in the endothelial dysfunction and the pathogenesis of atherosclerosis.