Solitary parasitoid wasps are believed to be able to adjust their oviposition behavior with host size. Host size theory shows that parasitoid's sex ratio is related to host body size, and parasitoids prefer to lay more female eggs in larger hosts. Superparasitism happens when solitary parasitoids lay more than one eggs in one host. And in the host superparasitized by solitary parasitoids, usually only one offspring can emerge. If survival frequency differs between the two sexual offspring in the superparasitized host, the relationship between adjustment of oviposition behavior by parasitoids and sex ratio in offspring would be affected. However, little has been known about the host size model when superparasitism has been excluded. Spalangia endius is a wide distribution solitary primary ectoparasitoid of the pupae of several Dipteral insects, most of which are important pest insects. The parasitoid has been used to test the host size model. Preview studies have showed that the parasitoid prefers to lay more female offspring in larger hosts, and a less female biased sex ratio in smaller ones. In this article, offspring number and sex ratio during oviposition, and the effects of host size and parasitism sequences on offspring sex ratio of S. endius were studied using the housefly pupae as the host. The results showed that the oviposition duration of the parasitoid was (8.93±3.34) days, and 34.11±16.34 females and 11.04±8.87 males were produced by a mother. Sex ratio in offspring was female biased, and the proportion of male was 0.24±0.11. Developmental duration of the male parasitoids was shorter than that of the females. Proportion of male in offspring increased with the increase of oviposition duration, which suggested a depletion of sperm in the older females. Larger hosts were preferred when large and small hosts were available together. The length of hind tibia of males was smaller than that of females. Some fly pupae still emerged houseflies when superparasitism had been excluded and only once parasitism behavior had occurred. Body sizes of housefly pupae which produced female and male parasitoid adults were (11.38±3.20) mm³ and (9.02±3.06) mm³, respectively, and hosts sizes produced male parasitoid offspring were significantly smaller than those produced female offspring. Our results illuminated that the parasitoid preferred to lay more female eggs on larger hosts when superparasitism had been excluded.