Lettuce was a leaf vegetable with short growing term and cold-tolerant plant. We grew it the whole year. The price of lettuce is the most expensive from July to September. But damages by lots of insect pests appear to greenhouse grown lettuce in summer season. In this study, we observed biological controls of western flower thrips(Franklinella occidentalisi) by the predator mites, Amblyseius swirskii and Neoseiulus cucumeris in lettuce greenhouse from July 7 to September 10. Hypoaspis aculerfer larvae and adults were released 30 individuals per m2 at the first one time as lettuce was planted on the greenhouse. We checked the density of F. occidentalis every 10 days on the 30 lettuce leaves. When the population of F. occidentalis reached 1~5 individuals per a leaf, A. swirskii larvae and adults were released 35 individuals per m2 at 1~3 times and Neoseiulus cucumeris larvae and adults also were released 150 individuals per m2 at 2~3 times in each lettuce greenhouse. During the high temperature season, Amblyseius swirskii was more effective biological controls of F. occidentalis than Neoseiulus cucumeris.