The parasitism preference of Diadegma semiclausum Hellen to different larval instars of diamondback moth (DBM), Plutella xylostella (Linnaeus) and four development ages of its 4th instar, and the effect of parasitism on the development and food consumption of DBM were studied in the laboratory at 24[degree]C. The results showed that D. semiclausum could parasitize 2 - 4 instars of DBM larvae, but preferred to 2nd and 3rd instars when the 2nd, 3rd and 4th instars of host larvae were exposed simultaneously to this parasite. The preference indexes to the 2nd, 3rd and 4th instars were 0.37, 0.44 and 0.19, respectively. The parasitic ability of D. semiclausum to the 4th instar DBM decreased with the increase of developmental age in this instar. The duration of prepupal and immature stages of the parasitized DBM larvae prolonged significantly. The host larvae were parasitized initially at the 2nd instar consumed less food than those unparasitized. However, there were no significant difference in the total food consumption between the larvae parasitized initially at the 3rd or 4th instar and those unparasitized. none