An assembly mechanism for assembling an annular workpiece to a fixture, includes a base seat, a fetching assembly, a guiding assembly, and a releasing assembly. The base seat defines a feeding hole and an uploading groove. The fetching assembly includes a first driving member and fetching subassembly. The fetching subassembly includes a sliding member and a fetching member, the handing member defines a fetching hole. The guiding assembly includes a guiding rod. The releasing assembly latches with the handing member. Wherein the first driving member drives the sliding member to slide from the uploading groove to the feeding hole, thereby fetching the workpiece from the uploading groove to the feeding hole, the guiding rod extends into the workpiece to position it, the releasing assembly releases the workpiece, then the workpiece is blown away from the guiding rod and is positioned on a top of the fixture.