The effect of parasitism in aphid population growth depends on the pattern of host instar selection. Patterns of preference may vary with the physiological state, including egg load, age and also size of female foragers. In this study, the effect of age and size of Lysiphlebus fabarum (Braconidae: Aphidiinae) females (asexual population) on host instar preference (Aphis fabae Scopoli), were studied. For this purpose, a cohort of females (1-day old, n=30) were introduced separately to 30 black bean aphid including (10 1st, 10 2nd, and 10 4th instar aphids). After 2 days, these females were introduced again, into host patches with previous conditions. In other test, small and large wasps (n=30) were introduced to 30 host aphids, with previous conditions. At the end of two tests, number of mummies formed, emergence rate and proportion of emerged offspring were determined. According to analysis, there was significant difference in number of mummies and proportion of emerged adults in fourth instar nymph between mothers with 1-day and 3-days old, such that in 1-day females were more than 3-days old ones. There was difference significant in emergence rate of offspring between small and large wasps at first and fourth instars. Furthermore, in small and large wasps, the number of mummies and proportion of emerged adults on first instar nymph was more than other instars significantly, whereas maternal size had no effect on host preference. This study showed that L. fabarum females in both ages and sizes, preferred first instar nymph than other stages.