The single crystal trigonal selenium nanoplates have been prepared directly from the trigonal selenium nanoparticles in a simple way with ethylenediamine in existence. The as-prepared nanoplates were characterized by XRD, TEM, HRTEM, SEM, and SAED measurements. It is shown that the selenium nanoplates have a single crystalline structure with the length and width reaching up to several microns while their thickness being only about 50 nm. This preparation method may be extended to the fabrication of other 2D elemental crystals with the similar crystal structures of trigonal selenium.