【Background】 The efficient use of parasitoids relies on knowledge of the biological and ecological relationship between parasitoids and hosts. Female wasps prefer to lay eggs on the most suitable hosts for the development of their offspring. 【Method】 The effects of host size on oviposition and development of Spalangia endius, the endoparasitoid of Bactrocera dorsalis, were studied in the laboratory. The host pupae of B. dorsalis were manipulated by starvation at larval stages to obtain host pupae with wide variation in body size. 【Result】 The parasitoid S. endius preferred to lay eggs in medium size host pupae, but more females and larger sized offspring were produced from larger host pupae. The body size of female offspring increased with host body size in a linear manner， but neither development time nor longevity of female or male offspring changed with host size. 【Conclusion and significance】 The results suggest that B. dorsalis could allocate its offspring number and sex ratio, so as to maximize its fitness, based on an assessment of host pupal mass.