In this study, the conversion efficiency (CE) of a core/shell quantum dot (QD) with type-II structure is studied by the detailed balance model. In particular, a size dependent strain effect caused by the lattice mismatch is considered since the CE under the detailed balance model is strongly dependent on the band gap of materials. The results show that the strain effect has a very important influence on the CE, and the multiple exction generation (MEG) can significantly increase the CE, so it must be considered in such a low-dimensional system.