Abstract: A centrifugal fan includes a casing and an impeller module. The casing defines an accommodating space having an air inlet and an air outlet. The impeller module is mounted in the accommodating space for allowing air flow to enter from the air inlet and exit from the air outlet. The impeller module includes a body and a plurality of blade units connected to and annularly distributed along a circumference of the body. Each blade unit has a plurality of blades arranged equi-angularly. A blade spacing which is defined between each adjacent pair of the blades of one of the blade units is different from a blade spacing which is defined between each adjacent pair of the blades of another one of the blade units.