For mounting in a host on a motherboard over memory modules in insertion slots and cooler modules on CPUs on the motherboard to guide currents of air in carrying waste heat from the cooler modules and the memory modules to the outside of the host, an adjustable air director is disclosed to include a main air duct shell, which has sliding tracks on the top wall thereof, and a supplementary air duct shell, which is attached to one lateral side of the main air duct shell and has sliding coupling devices respectively coupled to the sliding tracks and movable with the supplementary air duct shell relative to the sliding tracks to adjust the transverse width of the adjustable air director subject to the arrangement of the CPUs and the insertion slots on the motherboard.