Electron transfer rate is commonly calculated from well-known Marcus formula. As the electronic coupling strength becomes strong comparing to the reorganization energy, however, Marcus formula becomes inapplicable. Here, we overview the generalized non-adiabatic (NA) transition state theory (TST) for electron transfer rate proposed by us to overcome the weak electronic coupling limit. In the generalized NA-TST, the seam surface is adopted to get the thermally averaged nonadiabatic transmission probability based on Zhu-Nakamura formulas which cover from weak to strong electronic coupling regimes. In addition, we discuss the possibilities to introduce an effective one-dimensional reaction coordinate from the multi-mode donor-acceptor systems to estimate electron transfer rate. The potential applications are also illustrated.