A fan frame structure includes a plastic casing and a metal fixing frame. The plastic casing and the metal fixing frame are provided for an axial flow fan or a blower fan. The plastic casing includes a bottom having a plurality of first fixing portions. The metal fixing frame includes a base with a shaft tube seat in a center thereof. The base includes a plurality of second portions each engaged with one of the first fixing portions. Since the plastic casing is made of inexpensive plastic material, the manufacturing costs are cut. The metal fixing frame provides sufficient structural strength and desired heat dissipating effect.