The present work provides a new insight into the high purity synthesis of zirconium diboride (ZrB2) powders and a method of controlling impurity during the synthesis process. The single phase ZrB2 nanopowder was synthesized by a combined ball milling and carbothermal method using zirconium oxide (ZrO2), boron oxide (B2O3) and carbon (C) as starting materials. The reaction pathway, phase purity, and morphology of the ZrB2 produced are elucidated from X-ray diffraction (XRD) and scanning electron microscopy studies. The details of the impure phases generated during synthesis were obtained from multi-phase Rietveld refinements of XRD data. Experiments revealed that the method of synthesis carried out at 1750 degrees C involving ZrB2: B2O3: C at a molar ratio of 1:4.5:7.5 could produce highly pure ZrB2 nanopowders of 67 nm average crystallite size. The magnetometry studies on such pure form of ZrB2 nanopowders indicated that both paramagnetic and diamagnetic characteristics coexisted in ZrB2, which could be attributed to its polycrystallinity. (C) 2018 The Society of Powder Technology Japan. Published by Elsevier B.V. and The Society of Powder Technology Japan. All rights reserved.