Effectiveness of Encarsia formosa (Hymenoptera: Aphelinidae) to control Trialeurodes vaporariorum (Hemiptera: Aleyrodidae) was investigated in the commercial tomato greenhouse from January to June, 2010. Densities of T. vaporariorum were monitored using the yellow sticky trap for adult, and a lupe (x10) for 4th instar nymph and pupa at 1 week interval. Tomato seedlings were transplanted in mid-January and the nymph and adult of T. vaporariorum were first discovered in February 4th. E. formosa was applied as a mummy card at a recommended density (3 parasitoids per 1m2) at 2 weeks interval for 8 times. Mummy cards were collected 2 weeks later and the emergence and survival ratio of E. formosa were examined. Density of adult T. vaporariorum was significantly lower in the treatment plot (F=42.48, p=0.0001). The control efficiency ranged from 57.3% to 88.5%. The nymphal density was also significantly lower in the treatment plot (F=8.85, p=0.0053). The control efficiency was 84% on 25th March, 75% on 18th May and 95% on 25th May. Maximum 50% parasitism by E. formosa occurred in the treatment plot. The emergence and survival rates of E. formosa on mummy cardboards that were released in the greenhouse during February to March were only 53% and 41%, respectively. These rates increased to >70% as the temperature rises.