Abstract: Optical wave transmission through a subwavelength slit grooved with cavity in internal wall is investigated in a thick metallic plate by two-dimensional finite-difference time-domain method, and enhanced transmissions are found at some wavelengths in transmission spectra. Using Maxwell equations and waveguide resonance theory, the mechanism of the enhanced transmission is explored with amplitude distributions. It is considered that waveguide resonance modes play an important role in the transmission. The longitudinal modes forming in the subwavelength slit can lead to transmission peaks, and so can the cavity modes.