The effect of the grain structure produced by warm equal channel angular extrusion (ECAE) on twinning under subsequent room temperature deformation of magnesium alloy ZK60 was investigated. It was shown that a bi-modal grain structure, characteristics of which are determined by the ECAE temperature and the number of passes, has a strong effect on the tendency to room temperature twinning. The highest density of twins generated during room temperature deformation was found in material pre-strained by ECAE at 300 degrees C that exhibited a uniform recrystallised grain structure with an average grain size large enough for twinning to occur. (C) 2005 Springer Science + Business Media, Inc.