A molecular photodiode was fabricated with the hetero-Langmuir-Blodgett(LB) film consisting of an electron acceptor(A), an electron relay(R) and a sensitizer(S). TCNQ, viologen and flavin derivatives were used as A, R and S units, respectively. By aligning hetero-LB film of A/R/S units on ITO glass with an aluminum thin film, a molecular photodiode with the structure of Metal/Insulator/Metal(MIM) was constructed. Due to excitation by irradiation with a 460 nm monochromatic light source, the photo-induced unidirectional flow of electrons in the MIM device could be achieved and was detected as photocurrents. The direction of energy flow was in accordance with the energy level profile across the LB films. The photoswitching function was achieved and the rectifying characteristic was observed in the molecular device.