Formation of Pt + Cu and Pd + Cu alloy particles in acidic solutions at deposition electrode potentials which were located in the underpotential deposition region (upd) of Cu2+ ion was investigated by transmission electron microscopy (TEM). It was found that the behavior of the fraction of Cu in the alloy particles is similar to that of the Au-Cu alloy particles, that is, the fraction of Cu is independent of concentration of Cu2+ ion in solution but is dependent of deposition potential in the upd region of Cu2+ ion. This correlation can be described by an experimental formula. Unlike the case of the Au + Cu alloy particles, we did not find the multiply twinned particles (MTPs) of Pt + Cu alloy particles. The MTPs of Pd + Cu alloy particles were observed. However, these MTPs were not stable and changed to polycrystalline material quickly during the observation by TEM. Ag + Cu alloy particles were not formed in the upd region of Cu2+ ion.