Grooves play an important role in the transmission effidency of the metallic grating. The structure to excavate a groove in a cell of the period grating with one slit displays a dent in the transmission spectrum compared with the structure with only a slit in the cell. Increasing the depth of the groove can move the position of the dent in the tranmission spectrum to the longer wavelength direction. An analytical equation is also provided to approximately locate the dent.