The transmission property of a single thin metallic slit with different widths has been investigated by terahertz (THz) time-domain spectroscopy. The experiment results show that in the effective spectroscopic range (02—26THz), the Fabry-Perot-like behavior was observed when the slit width decreases to a certain value. According to the theory of surface plasmon subwavelength optics, this resonant phenomenon was analyzed.