Abstract: At the temperature of 26 ℃, effects of host age on the development and reproduction of Diachasmimorpha longicaudata(Ashmead), reared with the larvae of oriental fruit fly, Bctrocerat dorsalis（Hendal）, was investigated from the aspects of the parasitical efficiency，immature survival rate, wasp body size and sex ratio of its offspring, respectively.The results showed that host age had an obvious influence on the development and reproduction of D. longicaudata. Female wasps preferred to parasitize the 2^nd instar and the initial 3^rd instar larvae of oriental fruit fly, and the number of 63.00 and 85.50 host larvae parasitized at the corresponding ages were observed within 6 hours. But only 35.60 and 33.21 host larvae were parasitized when provided with the 1^st instar and the old 3^rd instar oriental fruit fly larvae, respectively. Development rate of D. longicaudata decreased with the increase of rearing host age. The generation development of D. longicaudata were 18.5, 18.9, 19.6 and 22.0 days when reared with the 1^st instar, 2^nd instar, initial 3^rd and old 3^rd fly larvae, respectively, and the corresponding immature survival rates were 16.40%, 46.88%, 54.09% and 26.29%, respectively. The development of D. longicaudata was greatly blocked at the 1^st instar and old 3^rd oriental fruit fly larvae. Body size of offspring wasps mainly depended on host’s ages, and the smallest wasps were observed from those 1^st instar oriental fruit fly larvae. However, a female-based sex ratio of offspring wasps was only exhibited in the initial 3^rd instar oriental fruit fly larvae, and the ratio of female offspring wasps was 64.11%.