A Mg95.5Y3Zn1.5 alloy processed via a two-step processing route of extrusion plus ECAP has been investigated. It was found that the ECAP processed Mg95.5Y3Zn1.5 alloy contained ultrafine grains and exhibitedexcellent mechanical properties. After ECAP, the average grain size of Mg95.5Y3Zn1.5 alloy was refined toabout 300 nm. The highest strengths, with yield strength of 444.6 MPa and ultimate tensile strength of472.7 MPa, were obtained after 1 pass at 623 K. The SAED patterns indicated that the microstructure afterECAP consisted of both high angle and low angle grain boundaries. The fraction of high-angle boundariesincreased with increasing numbers of ECAP passes. The Mg95.5Y3Zn1.5 alloy contained a high volume fractionof X-Mg12ZnY phase due to high yttrium and zinc addition. And, it accelerated the growth and coalescenceof cracks during tensile testing, resulting in premature fracture and lower elongation of alloy.