The emerald ash borer (EAB), Agrilus planipennis Fairmaire (Coleoptera: Buprestidae), (= A. marcopoli Obenberger), is an important bark beetle attacking ash trees (Fraxinus spp.). It is very difficult to detect and control because of its highly concealed life history. This pest mainly distributed in partial Asia countries (China, Japan, Korea, Mongolia) and Far East Russia, while in China it presented in Heilongjiang, Jilin, Liaoning, Shandong, Inner Mongolia, Hebei, Tianjin, Taiwan, etc. The important timber species F. mandshurica and gardening tree F. velutina were damaged severely in northern China. Spathius agrili Yang (Hymenoptera: Braconidae) is an important ectoparasitic wasp of the EAB larvae. This parasitoid has the potential to use as an excellent biological control agent for suppressing populations of EAB. We studied the differences of emergence date between overwintered S. agrili and its host, parasitism rates at different periods, relations between parasitism rates and host densities, and relationships between ovipositions of the braconid wasp and body sizes of host larvae using methods of regular surveys in forests and observations in laboratory. Results revealed that the emergence of S. agrili was more than one month later than that of its host, suggesting good synchrony between parasitoid emergence and host availability. The overwintered S. agrili emerged from mid June to mid August with the peak in July in 2003, and it was lasted from late May till late July with the peak during late June to early July in 2004. While the EAB emergence period ranged in mid-late May in 2003, and in mid April through mid May in 2004 (in laboratory). The emergence date of parasitoid asynchronously inosculated with the optimum developmental instars of the earliest host larvae, which was the results of a long-term co-evolution between the two species. It was also suggested that the parasitoid S. agrili could be a host_specificity parasitoid of EAB. The natural parasitism rates gradually increased in field on the whole at different periods. The body sizes of host larvae, i.e. larval instar, affected the decision of parasitoid S. agrili to lay eggs or not. Under natural conditions, this parasitoid usually oviposited only on those host larvae with a prontum and body width more than 1.5 mm, and a body length in excess of 12 mm, i.e. the 3rd to 4th instar larvae. But egg number deposited by female wasps showed no significant differences between. These findings would consequentially contribute to the further successfully biological control of the trunk borer.