An expandable frame structure includes at least one set of a transverse plate assembly and at least two vertical frames. Each set of the transverse plate assembly includes at least two transverse plate parts. Each transverse plate part includes a quarter circular arc edge and two side edges spaced and spreading out from each other with 90 degrees in angles. Two lower hook fixing parts extend downward from two side edges, respectively. Each of the vertical frames includes a pair of vertical frame parts and at least two horizontal frame parts. So, each of the transverse plate parts is placed and installed onto the horizontal frame parts of the two vertical frames. The lower hook fixing part is inserted into and engages with the horizontal frame part to form the expandable frame structure. The advantages and effects are easy to assemble and flexible for structural assembling styles, and so on.