Dynamic mechanical relaxations of La-based metallic glasses were investigated by mechanical spectroscopy. In the framework of the mixing enthalpy of constituent atoms, it was found that ß relaxation was less evident by the addition of Cu to replace Ni in the LaCuNiAl glassy alloy. By introducing Cu into the LaNiAl metallic glass, the mixing enthalpy was less negative, which led to weaker ß relaxation of the metallic glasses. The a relaxation of the La-based metallic glasses could be described by a Kohlrausch-Williams-Watts (KWW) function with a Kohlrausch exponent ßKWW around 0.5. It should be noted that physical aging above the glass transition temperature Tg induced a decrease of ß relaxation intensity in the La-based metallic glass.