Dense (>98%) submicron sized monolithic zirconium diboride (ZrB2) was fabricated by reactive spark plasma sintering (RSPS) of 8 h ball milled Zr and B elemental mixtures at 1200 °C. During RSPS, ZrB2 formed in-situ and the densification takes place by a two-step process. Plastic flow of ZrB2 resulting from application of higher pressure enhanced densification significantly. Improvement in nanohardness, elastic modulus and indentation fracture toughness were observed with increased sintering pressure. © 2015 Acta Materialia Inc.