Ultra fast solid-phase synthesis of nanostructured metastable zirconia of tetragonal modification from the amorphous hydroxide with no cation and anion promoters and from the zirconia of monoclinic modification was performed at the ambient temperature by mechanical stimulation of the reactions of dehydration, crystallisation and solid-phase transition in a high-energy centrifugal planetary ball mill. The process occurred by stages as the mechanical loading increased and crystallisation of metastable form from dry powder of precursors was accomplished in 15 min. Short-term mechanical activation of the amorphous zirconium hydroxide allowed also subsequent thermal crystallisation into the metastable zirconia to occur at low temperature (320degreesC). The generation of nano-sized zirconia is suggested to be one of the factors favouring the formation of the metastable form.