A centrifugal fan comprises an impeller and a casing. The impeller includes a hub and a plurality of blades. The casing comprises an upper casing, a lower casing, at least one protrusion, a first inlet opening and a second inlet opening. The lower casing is disposed opposite to the upper casing and connected to the upper casing to form an accommodation room for receiving the impeller. The protrusion is formed on the upper casing and/or the lower casing inside the casing and immediately surrounds the blades. The first inlet opening is defined on the upper casing and comprises a primary inlet opening augmented by a secondary inlet opening. The second inlet opening is defined on the lower casing. The protrusion surrounds the first inlet opening and/or second inlet opening.