A metal nanostructure sensing method for monitoring organic vapors is presented. A kind of triangular silver nanoprisms array with relatively high refractive index sensitivity is designed and analysed by finite difference time domain numerical method, and it is fabricated through nanosphere lithography fabrication method. Experiment results show that the fabricated nanostructures array is so sensitive to ethanol molecules that the detecting sensitivity reaches as high as 24ppm/nm. This organic vapor sensing method is expected to have a great potential in environmental monitoring.