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We prepared a series of Mg1–xZrxB2 bulk samples with different x values by the solid-state reaction at ambient pressure and investigated the effects of the Zr doping on phase compositions, microstructure, and flux pinning behavior. The results indicate that the a axis increases with the increase of the Zr doping and the c axis remains unchanged. ZrB2 phases form and a small amount of Zr atoms may substitute Mg in Zr-doped samples. Critical temperature drops with increasing the Zr doping. It is found that critical current density Jc can be significantly enhanced by the Zr doping and the best data are achieved in Mg0.9Zr0.1B2. For this sample, Jc values are remarkably improved to 1.83×106 A/cm2 in the self field and 5.51×105 A/cm2 in 1 T at 20 K. Also, the irreversibility field is increased by the Zr doping. The high density, fine grain size of MgB2, and small ZrB2 particles in Zr-doped samples may be responsible for the enhancement of Jc. Also, the field dependence of Jc at different temperatures was briefly discussed. ©2002 American Institute of Physics.