An exemplary elevating mechanism used for a flat-panel display monitor includes a support structure, a sliding structure, and a pneumatic cylinder. The support structure includes a pair of guide rails. The sliding structure is mounted to the support structure and is capable of sliding along the guide rails. The pneumatic cylinder includes a cylinder body, a piston shaft, and a first piston. The cylinder body is connected to the support structure. The piston shaft includes a first end received in the cylinder body, and a second end opposite to the first end connected to the sliding structure. The first piston is positioned in the cylinder body and partitions the cylinder body into two chambers. The two chambers are filled with gas. The first piston is fixed to the first end of the piston shaft and defines a through hole for communicating the two chambers.