An ultra-small on-chip long-wave photodetector by utilizing the thermal resistance effect of the metal strip on a hybrid plasmonic waveguide structure. When light propagates along a hybrid plasmonic waveguide, the metal strip is heated due to the light absorption. Accordingly, the resistance of the metal strip increases due to the thermal-resistance effect of metal and the resistance change can be measured accurately with a Wheatstone bridge so that the photode- tector is with very high sensitivity. Owing to the nano-scale confinement of light in a hybrid plasmonic waveguide, the temperature increase of metal due to the metal absorption is enhanced in comparison with the conventional long-range plasmonic waveguide. A theoretical responsivity of as high as 74 mV/mW is obtained with a low bias voltage of 1 V. The photodetector also has a fast response (< 1 µs) and works in a very broad wavelength range (e.g., 1.5 ∼ 7.5 µm).