An ultrafine Al particle consists of an Al multiply twinned particle. The Al multiply twinned particle has a decahedron structure surrounded by {111} planes. The Al multiply twinned decahedral partucle has a diameter of 10 to 30 nm. Such an ultrafine Al particle consisting of the Al multiply twinned decahedral particle is obtained as follows. A metastable Al oxide particle is placed on an amorphous carbon substrate having the reduction effect. Then the electron beam is irradiated to the metastable Al oxide particle placed on the amorphous carbon substrate in the vacuum atmosphere. From the metastable Al oxide particle, Al atoms or Al clusters are emitted and adsorbed to the substrate. By adjusting the electron beam intensity so that the ultrafine Al particle in the above procedure has a diameter from 10 to 30 nm, the Al multiply twinned particle having a decahedron is obtained.