The superplasticity and microstructure of magnesium alloy ZK40 processed by ECAP for I and 4 passes were investigated. Tensile experiments at 523 K showed that the elongation of ZK40 was improved with the increasing passes. The elongation-to-failure of ZK40 processed by ECAP for 4 passes tested at 523 K and strain rate of 1X10(4)S(-1) was 660%. The microstructural analyses explained these results.