A semiconductor device includes a bottom electrode on a substrate, a magnetic tunneling junction (MTJ) on the bottom electrode, a first cap layer on the MTJ, a second cap layer on the first cap layer, a block layer on the second cap layer, and a top electrode on the block layer. Preferably, the block layer could be made of Co-based alloy or metal nitride, in which the Co-based alloy could further include CoW alloy whereas the metal nitride could include WN.