In this study, we investigated the microstructures and high temperature plasticity behaviour of an Mg-5(mass%) Sn alloy after equal channel angular extrusion (ECAE). These results show that the grain boundary of the as-cast Mg-5(mass%) Sn alloy contained continuous eutectic α-Mg + Mg_2Sn precipitates. After an ECAE process, the average grain size decreased from 147 μm to about 10 μm and the continuous eutectic α-Mg +Mg_2Sn particles were broken down, and those particles distributed uniformly. The maximum elongation was 550 % at high temperature 350℃ with strain rate of 1 × 10^(-3) s^(-1).