A sheet size detecting mechanism includes an adjustable module, a sensing module and a controlling unit. The adjustable module includes a first sheet guide, a second sheet guide, an adjustable part and a circular gear. The adjustable part is a moved to a position where the first sheet guide and the second sheet guide are respectively in contact with two parallel edge sides of a sheet. The circular gear is rotated as the adjustable part is moved. The sensing module includes a sustaining element and a circuit board. The sustaining element is connected to an underside of the circular gear, and synchronously rotated with the circular gear. When the circular gear is rotated by an angle, the sustaining element is in contact with a specified one of the plural sensing regions, so that the circuit board generates a sensing signal. According to the sensing signal, the sheet size is acquired.