Abstract: Co-11at%W films with different thicknesses (9~80 nm) were deposited on MgO (220) single crystal substrate with Cr underlayer (100 nm) by magnetron sputtering. The epitaxial growth relationship between Cr and Co-W layer is Cr(112)[111¯]//Co-W(101¯0)[1¯21¯0] and Cr(112)[11¯0]//Co-W(101¯0)[0001]. With the increasing of Co-W film thickness, the in-plane strain of Co-W layer decreases from-0.388?4% to -0.2711%, while the out-of-plane strain decreases from 0.7813% to 0.5445%, and the first order effective magnetic anisotropy constant K1^eff decreases from 3.82×10⁶ erg/cc to 2.58×10⁶ erg/cc correspondingly. The results showed that the effective magnetic anisotropy energy of magnetic layer can be tuned by adjusting the strain between underlayer and magnetic layer.