An electron beam of more than 1 x 10 19 e/cm 2 .sec is irradiated upon metastable metal oxide particles such as Θ-Al 2 O 3 particles 1 disposed on an amorphous carbon film. A phase transformation of the metastable metal oxide particles results from the electron beam irradiation. Thus, stable metal oxide ultrafine particles such as α-Al 2 O 3 ultrafine particles 2 whose diameter is smaller than the metastable metal oxide particles used, can be produced. Metal nanoparticles of a metal component of the oxide such as of aluminium may also be produced.