An automatic focusing mechanism (100) is suitable for a digital camera in a portable electronic device. A preferred embodiment of automatic focusing mechanism includes a barrel (1), a holder (2), an optical module (3), and an image sensor structure (4). The barrel receives at least one lens (10) therein. The holder receives the barrel and the image sensor structure therein, and the thickness of the optical module can be changed for focusing. The optical module is located between the barrel and the image sensor structure. As the thickness of the optical module can be changed for achieving the purpose of focusing, the thickness and weight of the digital camera can be decreased.