A fan frame structure includes a base portion and a wall portion. The base portion has at least one extension section vertically extended from an outer peripheral edge of the base portion, and the extension section has connecting sides. The wall portion is formed along the outer peripheral edge of the base portion through an insert molding process. The wall portion includes a plurality of wall segments connected to the connecting sides of the extension sections, and a clearance is formed between any two adjacent wall segments. With these arrangements, the base portion can better resist a wrapping force from the insert-molded wall portion to avoid deformation.