Two types of transmission resonances are investigated with main interest centering on the transmission resonance condition. One is the transmission resonance through the Bethe’s small hole in the waveguide screen. The other is the transmission resonance through the 2D planar square array of the Bethe’s small holes. As a common feature between two structures, Rayleigh type Wood’s anomaly is observed to be associated with the transmission resonance phenomena for the two cases. In particular, the physical reason for the transmission resonance is explained by use of the image array concept in the parallel configuration in connection with the TM12 mode. That is, the transmission resonance frequency is mainly determined by the onset frequency of the lowest order grating lobe of the image array over the hole along the E-plane direction of the incident TE10 mode.