Abstract: Pea aphid Acyrthosiphon pisum (Harris) at different growth stages were used to explore the host stage preference of Aphidius ervi Haliday to pea aphid and the effect of host-stage parasitism on parasitoid offspring fitness at (24±1) ℃. The results suggested that the host stage of pea aphid had a significant (P < 0.05) influence on the parasitic behavior and fitness of A. ervi offspring. Although A. ervi parasitized on all the stages of pea aphid, the parasitic rate of A. ervi in the 2nd instar aphids was highest (56.67%). This was followed by the parasitic rate in the 1st instar aphids (30.67%) and the 4th instar nymphs, while that in the adults was least. The eclosion rate of offspring of A. ervi was significantly higher in young nymphs than in older hosts. The duration of offspring wasp was longest in the 1^st instar aphids. The duration from egg release to mummy appearance of A. ervi in the 1st instar aphids was 9.47 d and that from mummy appearance to eclosion was 5.51 d, which gradually shortened with increasing host stage. The duration of offspring development was longest in the 1^st instar aphids (14.85 d) and shortest in adults (7.87 d). The proportion of female offspring of A. ervi was significantly greater in adults (57.33%) than in 1^st instar (24.52%). Meanwhile, emerging adult size of offspring increased with increasing host stage (or body size). Host stage (or body size) was positively correlated with parasitoid offspring fitness. From offspring fitness measures, A. ervi females preferred to parasitize in older aphids for higher fitness return despite the risk of lower parasitic rate trade-off. The results suggested that host stage (or body size) significantly influenced parasitic selection of A. ervi and offspring fitness.