Some developmental characteristics of a larval-parasitoid ichneumonid, Diadegma semiclausum Hellen, donated from Asian Vegetable Research and Development Center (AVRDC), Taiwan, in 2001, were investigated to utilize as a biological control agent of diamondback moth (DBM), Plutella xylostella L. As a preliminary test for rearing the prey insect, we selected the most preferable host plant for oviposition. Among five host plants tested, DBM female laid the highest number of eggs on the aluminum foil prepared with radish leaf juice, followed by rape, Chinese cabbage, broccoli and common cabbage. The developmental periods for the whole larval stage of D. semiclausum and DBM were 12.3 days and 14.2 days at $23^{circ}$ , respectively. Also, parasitoid cocoons could be stored at $8^{circ}$ for 24.5 days without a significant loss of viability. The DBM pupa, however, lasted for approximately four weeks at $8^{circ}$. The percent parasitism to DBM was 70.8% on average, and the sex ratio of D. semiclausum adults was slightly male-biased, 0.94:1. The seasonal fluctuation of DBM density showed apparently two generations, and the peak periods occurred in middle June and middle July.