A fan includes a housing and a motor. The housing has a compartment, at least one lateral air inlet and at least one lateral air outlet. The lateral air inlet and lateral air outlet communicate with the compartment. The motor has a stator and an impeller. The impeller is coupled with the stator and includes a hub and a plurality of blades coupled with a top face of the hub. Each blade has first and second ends. The first ends of the blades face outwards, and the second ends of the blades define an air-guiding room encompassed by a virtual geometric pattern formed by a virtual line linking the second ends of the blades. The virtual geometric pattern has a maximal diameter. A gap is formed between the second ends of two adjacent blades. The gap is smaller than the maximal diameter of the virtual geometric pattern.