Temperature- dependent, biological aspects longevity and parasitism of the parasitoid Diaeretiella rapae (M'Intosh) was measured at three constant temperatures (15, 25 and 30 ^oC) using Brevicoryne  brassicae (L.) .  The threshold (t₀) and thermal units (DD's) of different stages of D. rapae at different temperatures, 15, 25 and 30 ^oC., were studied. The rate of development of all stages increased as the temperature increase from 15 to 30 ^oC. The mean of thermal units recorded 117.9, 88.84 and 222.45 DDs for egg-mummy period, mummy- adult period   and adult longevity respectively. Also, the threshold development estimated as 2.4, 1.6 and 3.4 ^oC for egg- mummy period, mummy- adult period and adult longevity respectively.