The present invention provides an assembly unit of frame structure which comprises a column body, a cover body, and a fastening element. By assembling a predetermined number of the assembly units to incorporate with a plurality of supporting frame through an easy and prompt assembly process, a frame structure of desired shape can be quickly and easily constructed for use. The frame structure can be construct by the assembly unit of the present invention into various shape, such as rectangular shape or right triangular shape, without any special tools or technology.