Abstract: As driven by external neutron source and working under subcritical state, the accelerator driven subcritical system (ADS) provides larger safety margin and is widely adopted for minor actinides (MA) transmutation. Loading MA into the subcritical core, especially the isotropic of americium, will decrease the magnitude of the negative Doppler feedback effect, increase the positive coolant temperature coefficient and also reduce the effective delayed neutron faction. Furthermore, it will impact the system response under transient processes. Based on an ADS transmutation design loaded with metallic fuel, the transient of beam overpower and unprotected transient of overpower accidents under different americium concentration conditions was investigated. It is found that the impacts of americium are not obvious. This demonstrates the enhanced safety characteristics and the potential flexibility to utilize fuel with high MA ratio.