A method of fabricating a semiconductor device includes forming a magnetic tunnel junction layer including a first magnetic layer, a second magnetic layer, and a tunnel barrier layer interposed between the first and second magnetic layers, patterning the magnetic tunnel junction layer to form a magnetic tunnel junction pattern, forming an insulating layer to cover the magnetic tunnel junction pattern, and performing a thermal treatment process to crystallize at least a portion of the first and second magnetic layers. The thermal treatment process may include performing a first thermal treatment process at a first temperature, after the forming of the magnetic tunnel junction layer, and performing a second thermal treatment process at a second temperature, which is higher than or equal to the first temperature, after the forming of the insulating layer.