In the experiment for studying the nature of coupling between suface plasmon （SP） wave and terahertz radiation by subwavelenth-sized brass rods, we found that the polarization of the THz field, size of rods, and slit between rods critically affect the coupling efficiency. By the method of Fourier transform of the time domain data, we found the coupling frequency in THz range. With equal slits between the brass rods, we demonstrated thar the transmission resonance is attributed to the combined effects of surface plasmon and cavity mode resonance when the electric vector is perpendicular to the grating wires, while the cavity mode resonance dominates the resonance transmission when they are parallel.