Spontaneous copper selenide formation was studied by the use of Cu particles which were partly deposited Se amorphous films. The selenide crystals were formed in both selenium films and Cu particles. If the surface of Cu particles were covered with Cu2O thin layer, the direction of Se atoms was changed by the Cu particle size. In large size (130nm), selenide was formed in both Cu particle and Se film. In small size (40nm), selenide was formed in Cu particle by the diffusion of Se atoms passed through the Cu2O layer.