The magnetic relaxation measurements were made using Zn doped YBCO single crystals YBa2(Cu1-xZnx)3O7-�. The decay of magnetization showed a linear dependence with the logarithm of time in wide temperature range of 0.1 < T/Tc < 0.8, which indicates a strong flux pinning in these samples effective up to higher temperatures of 0.8Tc. The temperature dependence of the normalized creep rate S(T) for applied fields of 1 T shows a non-monotonic behavior in the whole temperature range, with a peak at lower temperature and a dip at intermediate temperatures which is contrary to the predicted plateau in this region and indicates a strong change in the pinning mechanism of these samples at these temperatures and fields. The dip in creep rate S(T) of these samples up to 3 Zn concentration was found to be as large as 0.008, compared to 0.025-0.045 observed in the similar strong pinning systems. The weak temperature dependence of the pinning potential for these Zn doped samples at low temperatures shows drastic and systematic changes in U0 at higher temperatures which indicates that the Zn impurity contributes effectively to modify the pinning properties up to higher temperatures. © 2007 Elsevier B.V. All rights reserved.