We present light-induced switching mechanism of porphyrin-coated Si nanowire field effect transistors (Si NW FETs). Si NW FETs were fabricated by bottom-up methods and show ambipolar characteristics due to thermally annealed Schottky barrier on the contact between the electrode and NW channel. Si NW FETs are functionalized by porphyrin, a key dye molecule in photosynthetic process, to have photo-sensitive operation. Porphyrin-coated devices show clear current switching under light illumination that is not shown in bare devices. Switching time and switching current ratio depend on the concentration of porphyrin. Under light irradiation, electrical properties of molecular layer are changed; increased mobile charges by photo-excitation screen electrical field from the applied bias. In addition, molecules are polarized by charge separation that build vertical field towards the NWs. The electrical charge of porphyrin layer modifies the total applied field in NW, which induces current switching according to the concentration of porphyrin.