A cooling fan (20) includes a housing (22) and an impeller (24). The housing has a base (221), a sidewall (222) extending from a periphery of the base with an air outlet (226) defined therein, and a cap (223) covering the sidewall. The impeller has a plurality of blades (242) rotatably received in the housing for generating an airflow. An air channel (225) is formed between tip portions of the blades and an inner face of the sidewall. The air outlet has a near section (226A) and a far section (226C). The airflow first reaches the near section and then the far section. The base of the housing has a protrusion (227) extending into the air outlet for guiding the airflow flowing towards the far section from the near section of the air outlet.