A method for fabricating semiconductor device includes the steps of forming a magnetic tunneling junction (MTJ) stack on a substrate, performing an etching process to remove the MTJ stack for forming a MTJ, performing a deposition process to form a polymer on a sidewall of the MTJ, and removing the polymer to form a rough surface on the sidewall of the MTJ. Preferably, the MTJ could include a pinned layer on the substrate, a barrier layer on the pinned layer, and a free layer on the barrier layer, in which the rough surface could appear on sidewall of the pinned layer, sidewall of the barrier layer, and/or sidewall of the free layer.