Microstructure evolution and deformation mechanisms of AZ31 magnesium alloy tubes during bending have been investigated. Dislocation slip appears to be the main deformation mechanism, along with a few {10-12} [-1011] extension twins at the outer bend radius which undergoes tensile deformation. In contrast, the material in the tube wall at the inner bend radius of the tube, which undergoes compression, deforms mainly by extension twinning. This understanding of deformation mechanisms has explain the optimum bending temperature of 150-200 A degrees C for the AZ31 tubes where both slip and twinning are active.