Hightemperature deformation around the glass transition temperature T_g and the dynamic mechanical behavior of La₃₀Ce₃₀Al₁₅Co₂₅ metallic glass were investigated. According to dynamic mechanical analysis (DMA) results, La₃₀Ce₃₀Al₁₅Co₂₅ metallic glass exhibits a pronounced slow β relaxation process. In parallel, strainrate jump experiments around the glass transition temperature were performed in a wide range of strain rate ranges. The apparent viscosity shows a strong dependence on temperature and strain rate, which reflects the transition from nonNewtonian to Newtonian flow. At low strain or high temperature, a transition was observed from a nonNewtonian viscous flow to Newtonian viscous flow. It was found that the activation volume during plastic deformation of La₃₀Ce₃₀Al₁₅Co₂₅ metallic glass is higher than that of other metallic glasses. Higher values of activation volume in La₃₀Ce₃₀Al₁₅Co₂₅ metallic glass may be attributed to existence of a pronounced slow β relaxation. It is reasonable to conclude that slow β relaxation in La₃₀Ce₃₀Al₁₅Co₂₅ metallic glass corresponds to the "soft" regions (structural heterogeneities) in metallic glass.