The patterns of dislocations and their configurations of a ho t rolled Mg-3Al-1Zn (AZ31) plate were investigated at the stress amplitude of 90 MPa by using a tension-compression fatigue test at room temperature. The result shows that basal <a> dislocations were rearranged into pileups, dislocation walls and low-angle grain boundaries during high cyclic deformation. Due to continuously reversed loading along the rolling direction, <c + a> pyramidal dislocations were activated. <c + a> edge dislocations were straight lines with spot-like features and parallel to the basal plane trace. Zigzag dislocation were observed, due to the double cross-slip of the pyramidal dislocations between the first-order and second-order pyramidal slip systems. By interactions of basal-basal, basal-prismatic and pyramidal-pyramidal dislocations, junctions and dislocation loops are also observed.Keywords: Dislocation configurations; Magnesium alloy; High cycle fatigue