The sintering shrinkage of zirconium diboride, like that of other brittle materials, is the same in the direction of application of the compacting force and in the radial direction. At temperatures of up to 1700° C, there is a slight, practically linear dependence of shrinkage on holding time, which is due solely to the ordering of the grain boundaries. At sintering temperatures of 1800–2200° C and holding periods of up to 30 min, a pronounced shrinkage is observed, which is probably due to slip processes taking place at the grain boundaries under the influence of surface tension forces. At slight grain coarsening takes place at the same time. At high temperatures and holding periods greater than 30 min, shrinkage is brought about by viscous flow, which is due to a directional diffusional displacement of the atoms. The presence of dodecaboride in zirconium diboride activates shrinkage by accelerating diffusional processes along the grain boundaries.