Longan witches' broom is a very serious disease in Mekong delta and it is difficult for management. Long Nhung mite Eriophyes dimocarpi is proven for associating with longan witches' broom disease. This is a species which has a small size, and its population increases rapidly in a year. However, there were few studies that had been done before. Therefore, studying on ecology of E. dimocarpi such as distribution, host range and natural enemies is needed. The result of study showed that E. dimocarpimite was uniformly distribution according to directions of branches on longan trees, they appeared on un!inished maturity leaves (28.08 mite/ leaflets), mature leaves (19.10 mite/ leaflets), and young leaves (2.79 mite/ leaflets). At dry season (1st survey) Long Nhung mite appeared the most popular on Tieu Da Bo variety, and it appeared popular on Ido, Long Hung Yen, Long, Super, Vung Tau, Xuong Com Trang, rambutan Nephelium lappaceum (Sapindaceae). However, it was the least popular on Cui, Xuong Com Yang, NL19 and cassava Manihot esculenta (Euphorbiaceae). At rainy season (2nd survey) Long Nhung mite appeared on longan varieties, rambutan and cassava same in the dry season, except Super and Long varieties. The result of the survey of natural enemies of E. dimocarpi on Tieu Da Bo longan orchards in the Mekong delta recorded that larvae Arthrocnodax sp. (Cecidomyiidae, Diptera) was an predator of E. dimocalpL Arthrocnodax sp. appeared in low frequency (10 percent, 2012 and 20 percent, 2013) on investigated whole gardens.