Observation of water vapor pressure is very helpful for understanding the operation condition of a high vacuum system. In this work, we demonstrate a water vapor gauge based on the resistance change of gold nanoparticle film. The gauge has low detection limit (~10^(-4) Torr) and fast response (response time < 0.5 sec). We found that the response of heme molecule modified gold nanoparticle film and the water vapor pressure follow a power-law relation (γ ≈ 0.59). The calculated characteristic adsorption energy is about 22 meV.