The brassica leaf beetle Phaedon brassicae Baly, one of the pests of cruciferous vegetables in China and Japan, is a multivoltine species that over-winters and over-summers mainly as diapausing adults. In this paper, the internal reproductive system is described and illustrated for this species. Its morphology associated with diapause is also described. The accessory glands, lateral ejaculatory duct, common ejaculatory duct and ejaculatory sac are poorly developed in diapausing male, in contrast, these are obvious and swollen in the reproductive male. In the diapausing female, no obvious previtellogenic oocytes can be found in the ovariole. The ovarioles contain numerous previtellogenic oocytes, and the basal oocytes have visible yolk uptake in the reproductive female. In addition, the fat bodies are numerous and dark yellow in color for diapausing adults, but few and light yellow in color for reproductive adults.