A centrifugal fan includes a fan frame and a fan impeller received in the fan frame. The fan frame has at least one air inlet and at least one air outlet respectively formed on two sides of the fan frame. The fan impeller has a hub, an extension section outward extending from the hub and multiple blades disposed on the extension section. Each blade has a front end and a rear end. The rear ends of the blades are annularly arranged to together define a virtual geometrical configuration. A flow guide space is defined between the virtual geometrical configuration and the hub. A chord length is defined between the front and rear ends of the blade. The flow guide space has a width larger than or equal to the chord length so as to thin the centrifugal fan and enhance the performance thereof.