A fan includes a fan frame and an impeller. The fan frame includes a frame body and a base, and the impeller is accommodated in the main body and is disposed on the base. The frame body has a column-shaped passage, an airflow outlet and an airflow inlet. The main body further has two expansion portions respectively disposed adjacent to the outlet and inlet. The expansion portions extend from the column-shaped passage to the outlet and the inlet, respectively. The main body has at least two planar sides so as to allow parts of the expansion portions to respectively form straight-cut lines where the planar sides are located, respectively. The base has several straight-cut planes, and each of the straight-cut planes is located with respect to the straight-cut line.