Abstract: Field and laboratory studies in 1981-1986 showed that in endemic areas of mala-yan filariasis with Anopheles sinensis as the transmitting vector and in endemic areas of bancroftian filariasis with Culex pipiens pallens as the transmitting vector, when the microfilaria rate of the human population met thc criterion of elimination of filariasis, namely below 1% of thc total population in all the administrative villages, and the average density of microfilaria per 60/μ1 peripheral blood bcing around 5, the residual microfilaremia cases could turn negative one after another within 3-5 years, even if they did not receive causative treatment. Thc microfilaria rate of the human population would decline year by ycar, the natural infection of mosquitoes with filaria larva would fall down to zero, no new patent infection of filariasis occurred. The transmission was thought interrupted.