Abstract We report an anomalous and strong increase in magnetization found in Zn doped YBCO single crystals at intermediate fields and wide temperature range below critical temperature T c. As spinless divalent Zn preferably replaces Cu at CuO 2 planes, strong changes in the electronic and magnetic behaviour of these planes are expected with the increase of Zn doping. The magnetic hysteresis (M-H) loops were taken at a constant temperature using a SQUID magnetometer in 5 T magnetic field and in the temperature range 0.5≤ T/ T c≤0.94. It was found that with the increase of Zn concentration peak field H p, irreversibility field H irr, and critical current density J c, decrease substantially and systematically instead of increasing as predicted by collective pinning model for increased disorders. In the range of Zn doping from 1.5% to 4.3%, the large difference of the factor of 2.5 in peak field H p was found in the temperature range 0.7≤ T/ T c≤0.9. H irr also showed similar decrease with the increase of Zn in this temperature range. The decrease in the condensation energy and the increase of anisotropy with the increase of Zn content seem to have dominant role in the reduction of H p, H irr and J c. The normalized current density with the reduced field showed very slow response with the field above peak for higher Zn content samples compared to lower ones. This indicates the varying role of intrinsic parameters ( λ, ξ) for J c with different Zn concentrations. The normalized pinning force curves with the reduced field using Kramer's model in our samples showed reasonable scaling in the temperature range 0.7≤ T/ T c≤0.94, with the shift of peak from reduced field value of 0.31 to 0.25 with increasing Zn concentration. This reflects that the scaling is sample dependent. SEM observations showed higher density of twins with increasing Zn content. The changes in the intrinsic parameters and the increased structural defect density with increasing Zn content seem to be a possible cause of the onset of plastic flow of vortices at lower values of magnetic field and lead to strong peak effect in these samples.