The slow β relaxation which occurring at temperatures far below glass transition temperature, typically playing importantroles in the plastic deformation behavior of metallic glasses. Compared with polymer glasses, most of the metallic glasses donot exhibit obvious β relaxation based on dynamic mechanical analysis. In the current work, prominent β relaxation behaviorsof a series (LaxCe100−x) Al10Co25(x = 50, 60, 70, 80) bulk metallic glasses (BMGs) at low temperature are observed usingdynamic mechanical analysis. Compared with other BMGs, the LaCe based BMGs show a pronounced β relaxation peak andrelative low peak temperature, the activation energy of the β relaxation based on Arrhenius equation are calculated around79.3 kJ/mol to 86.4 kJ/mol, which obey an empirical rule that Eβ ≈ (26 ± 4) RTg, with the coefficient of 23. Experimentalresults from room temperature compression tests for the LaCe based BMGs indicate that low temperature activated β relaxationbehaviors facilitate the good plasticity of the LaCe based BMGs.