The microstructure and mechanical properties of AZ61-4Si magnesium alloy before and after equal channel angular processing (ECAP) were studied. The results show that Chinese script type Mg2Si phase changes into fine granules after 4-pass ECAP. The matrix α-Mg and divorced eutectic β-Mg17Al12 are also refined. The yield strength, elongation and tensile strength are increased by 128%, 340% and 89%, respectively. The high temperature creep fracture life increases by 8 times and the creep rate is 0.058%/h. Compared with the 4-pass ECAP, the microstructure shows little change after 8-pass ECAP. The yield strength and creep rate of the 8-pass ECAPed alloy are improved while the yield strength, elongation and high temperature creep fracture life are reduced. The modification mechanism for microstructure and mechanical properties of the experimental alloy by ECAP was analyzed