Abstract: A fire damper having a duct, a flap rotatable about an axis crossing the duct, an engaging member, and a spring ordinarily urging the flap to rotate in a direction. The fire damper has a trigger mechanism comprising a leaf spring fixed to the flap and having a free end engageable with the engaging member so as to stop the flap at an angle parallel with air flow. The fire damper further has a heat detecting and actuating member made of form memory alloy spring which has an end fixed with the flap and has another end fixed to the leaf spring at a position near the free end of the leaf spring.