We investigate the ultrafast magnetization dynamics of perpendicularly magnetized MgO/CoFeB/Ta and Ta/CoFeB/MgO stack structures using an all-optical time-resolved magneto-optical Kerr measurement technique. We find that the effective Gilbert damping parameter alpha(eff) converges to a low value of similar to 0.02 in the high external field regime, and it stays unchanged at similar to 0.02 regardless of the pump fluence. Two asymmetric stack structures of CoFeB show identical dynamic behavior and Gilbert damping, implying that the influence of the under and top interfaces is not distinct. (C) 2015 Elsevier B.V. All rights reserved.