Abstract: This paper deals with the infection index of Anopheles sinensis infected with the blood meals of 9 different microfilarial densities (5-300 mf/10μl) of periodic Brugia malayi. The results indicated that the mean number of microfilariae (mf) ingested by mosquitoes increased with the mf-density of the blood meal. The infection rates of vectors were 30, 65, 93 and 100%, when mf-densities were 5, 10, 20 and 50 mf/10μl respectively. Although the infection rate was 100% when mf-densities were larger than 50(100, 150, 200, 250, 300) mf/10μl. the infection intensities were gradually increased from 17.2 to 51.4 and the host efficiencies were decreased from 0.4135 to 0.2328. The infection intensities and host efficiencies of low mf-densities (5, 10, 20, 50 mf/10μl) were 1.22-8.40 and 0.5669-0.6356 respectively. Under the condition of 27.5±0.5℃, RH75±5%, the developmental period from mf to third stage larva was 8 days and numerous infective larvae could be harvested when mf-density was 200 mf/10μl. The relationship between mf-density and host efficiency, number of melanized larvae and susceptibility of vector, (experimental infection indices of Kartman and of Wharton were discussed.