This is related to a fan including a casing, a first impeller structure, a second impeller structure, and at least one driving device. The casing has an outlet and an air-containing portion having an entrance and an exit provided inside the casing. The first impeller structure and the second impeller structure are installed inside the casing, and include a first blade set and a second blade set, respectively. The first blade set is located corresponding to the entrance of the air-containing portion. The second blade set is located corresponding to the exit of the air-containing portion. The driving device drives the first and second impeller structures.