Common sunn pest (CSP), Eurygaster integriceps Puton is the most important pest of cereals in Iran. Using biological control agents specially egg parasitoids (Hym., Platygastridae and Encyrtidae) against the pest has been promising. An optimal use of them, needs one beware of their efficacy at different ecological conditions. The Encyrtidae can play different roles in a cereal agroecosystem depending on level which it will occupy in food chain. It means it can appear either as primary parasitoid of CSP or hyperparasitoid of the platygastrids. Although at first glance hyperparasitism seems an undesirable property, but it depends mainly on if it is a selective behavior. Two species of parasitic wasps, are used in this study, Ooencyrtus fecundus Ferriere & Voegele (Hym.: Encyrtidae) and Trissolcus grandis (Thomson) (Hym.: Platygastridae). In this study host preference of O. fecundus was evaluated in different combinations of host (fresh vs. parasitized eggs by Trissolcus grandis (6d old); fresh vs. developed 8d old eggs; parasitized (6d old) vs. developed (8d old) eggs). Functional response experiments also were done at densities of 1, 2, 4, 7, 14, 28, 56 host eggs from any kind in 20, 15, 10, 7, 5, 3, and 3 replications respectively. The results revealed that the parasitoid definitely prefer the fresh eggs and the preference is not affected by relative densities of the hosts. When no suitable host was available, handling was declined considerably and negative effect of fetal development was stronger than previous parasitism. Therefore, O. fecundus prefers parasitized eggs to developed CSP eggs. The handling times as well as maximum attack rates in functional response experiments also confirmed the results of the preference experiments. These parameters were estimated to be 0.95 ± 0.04, 1.21 ± 0.09 and 2.49 ± 0.23 h for the former and 25.19, 19.71, 9.61 per day for the latter. It seems that the wasp partly saves its reproductive energy for subsequent fortunes in circumstances that the favorable host is unavailable.