Abstract: The air cap and diffuser assembly is for vent-registers connected to the air ducts or the trunk line of a central air conditioning system. The cap assembly is designed to cover an air duct opening or for redirecting air flow. The cap has a peripheral seal that presses against the rim face of the register by magnet and/or a fastening device, and is comprised of a male cap/diffuser and a female cap/diffuser of sufficient size to cover the opening of a ceiling register. They are easily assembled together by snapping it on to form a complete air cap assembly. Also, a male or female cap/diffuser of the assembly can be removed to switch the function to an air diffuser/flow redirector.