Abstract: A fan duct includes an immovable cylinder and a movable cylinder. The movable cylinder is capable of sliding in an axial direction of the immovable cylinder. One of the immovable cylinder and the movable cylinder defines at least one groove in an axial direction thereof. Each groove defines at least two locating slots communicating with the respective groove. The other of the immovable cylinder and the movable cylinder forms at least one pin corresponding to the groove, the pin being capable of selectively sliding into any one of the locating slots. A related heat dissipation assembly including the fan duct is also disclosed.