A fixing mechanism, applied to an electronic device to fix an electronic module. The fixing mechanism comprises a casing, a fixing structure and a locking structure. The casing includes a baseplate and a side plate to define a space to accommodate the electronic module, and the side plate has a hole and a seat. The fixing structure includes a fixing element and a pressing portion. The fixing element movably extends from the outer side of the side plate into the space through the hole. The pressing portion is rotatably disposed on the combining seat, and includes a first part having an opening and a second part corresponding to the fixing element. The locking structure is movably set in the opening, and includes an engaging element. When the engaging element is engaged in the opening, the locking structure presses against the side plate to fix the pressing portion.