Host age is an important factor affecting the offspring sex ratio of egg and larval parasitoid wasps. Relatively few studies have investigated the effect of host age on the offspring sex ratio in pupal parasitoid wasps. The current study was undertaken to assess whether the age of host pupae could influence the offspring sex ratio of the pupal parasitoid Pimpla luctuosa Smith (Hymenoptera: Ichneumortidae: Pimplinae). Test female wasps were simultaneously provided with young and old host pupae of different sizes, and the effects of host age, size and the interaction between the two factors were examined. Both host age and size significantly affected the offspring sex ratio of P. luctuosa. The proportions of female offspring produced from hosts increased with host size in both host age classes. Old hosts produced fewer female offspring than did young hosts, however. No significant interactions were detected between host age and size. The results indicated that female P. luctuosa responded hosts of different sizes and ages by changing the offspring sex ratio. Besides, the mean sex ratios were highly male-biased, regardless of host age. The sex allocation strategy in P. luctuosa is discussed. Factors causing male-biased sex ratios are also discussed. none