Abstract: We report an ultrafast spectroscopy investigation that addresses the subtle location effect in a prototypical semiconductor-MOF hybrid system with TiO_2 nanoparticles being incorporated inside or supported onto Cu_3(BTC)_2, denoted as TiO_2@Cu_3(BTC)_2 and TiO_2/Cu_3(BTC)_2, respectively. By tracking in real time the interface electron dynamics in the hybrid system, we find that the interface states formed between TiO_2 and Cu_3(BTC)_2 can act as an effective relay for electron transfer, whose efficiency rests on the relative location of the two components. It is such a subtle location effect that brings on difference in photocatalytic CO_2 reduction using the two semiconductor-MOF hybrids. The mechanistic understanding of the involved interface electron-transfer behavior and effect opens a helpful perspective for rational design of MOF-based hybrid systems for photoelectrochemical applications.