The unusual optical transmission enhancement is observed during exploring the optical properties of the periodically
patterned metal films. We fabricate the nanohole array on gold film with the focused ion beam techniques. The
transmittance is higher than the ratio of the area occupied by the holes and the total etching area. The dependences of the
transmittance on the thickness of metal film and the radius of hole are obtained. Theoretical calculations show that
extraordinary optical transmission is due to coupling of surface plasmon modes on the same surface, as well as coupling
of modes on both sides of the metal film. The calculated data are in good agreement with the experimental results. After
the analysis of the phenomena, we predict that the periodically modulated metal film is useful in field of photoelectronic
detecting and imaging.