A fixing mechanism is applied to an interface card assembly and an electronic apparatus. The fixing mechanism includes a movable window and an operation component. The movable window is slidably disposed on a casing of the interface card assembly. The operation component has a fixed end and a free end opposite to each other. The fixed end is disposed on the movable window. The free end is detachably engaged with the casing to position the movable window. The movable window is positioned on one of a first region and a second region of the casing for respectively fixing interface cards with different sizes to the casing.