Magnetic measurements were made using pure YBCO and Zn doped YBa2(Cu1-xZnx)3 O7-�. Single crystals with Zn concentration of 0.5, 1.5, 3.0 and 4.3. The magnetic hysteresis loops for these samples were measured in the temperature range 0.1 � T/Tc � 0.96 under magnetic fields of 5 T using SQUID. It was found that the critical current density Jc increased for low Zn content samples up to 3 Zn concentration compared to pure YBCO sample and decreased for the higher Zn content samples. These values varied consistently when compared at magnetic fields of 1 T and 3 T. Moreover Zn doped samples showed significant values of Jc in the temperature range of 0.7-0.9Tc, close to critical temperature compared to pure YBCO sample. The irreversibility field Hirr was also enhanced in this temperature range showing consistent decrease with increase of Zn concentration. The peak field Hp above Hc1 and irreversibility field Hirr, both show power law dependence of the form H = m1(1 - T/Tc)m2 in the temperature range of 0.75-0.96Tc. The values of parameter m2 increased from 1.44 to 1.95 for the samples up to 3 Zn content and decreased to 1.37 for higher Zn contents. The ratio Hirr/Hp was found to be 3-4 for the lower Zn content samples and was 7-8 for the sample with high Zn content indicating more disorder for higher Zn content samples. The region between peak field Hp and irreversibility field Hirr was broadened with the increase of Zn concentration. The strong effect of Zn substitution in modifying behavior of these samples even at elevated temperatures is possibly due to the changes in the anisotropy of our samples with the increase of Zn concentration and also due to the locally induced changes in magnetic moments by Zn substitution. © 2010.