Abstract: Developmental time, adult longevity, temperature threshold and effective temperature of two strains of Encarsia formosa from Beijing and originally introduced from USA on Bemisia tabaci were studied at 18-33℃. It showed that the developmental time from egg to adult of both strains decreased significantly with the increase of temperature, which ranged from 34.7d at 18℃ and 7.8d at 33℃ for the USA strain, and 36.1d and 8.4d for the Beijing strain. Besides, the development rate of the USA strain was significantly faster than that of the Beijing strain at 18 and 23℃. Adult longevity of E. formosa were strongly affected both by temperatures and diets. The provision of honey water and/or 3rd-4th instars whitefly nymphs could prolong adult longevity, which was the longest at 18℃ and the shortest at 33℃. For the USA strain, the temperature threshold for the immature stage, adult stage and the whole generation were calculated to be 13.1, 12.3, 12.8℃, respectively. And the effective temperature sums for these stages were assessed as 177.23, 179.11 and 353.53 degree-day, respectively. For the Beijing strain, these were 3.2, 11.4, 12.4℃ and 188.25, 168.58 and 358.28 degree-day, respectively.