Microstructure and mechanical properties development of extruded Mg-1Y (wt. %) binary alloy during equal channel angular pressing (ECAP) with route B c at 400 • C, and subsequent annealing treatment between 300-400 • C at different holding time of 5-120 min were investigated using an optical and scanning electron microscope (SEM), electron back scattered diffraction (EBSD), tensile test, and hardness test. The grain size of as-extruded material (~10.9 µm) was refined significantly by 1-pass ECAP (~5.8 µm), and resulted in a remarkably enhanced elongation to failure (EL) (~+62%) with a slightly decreased ultimate tensile strength (UTS) (~−3%) comparing to the as-extruded condition (EL = 11.3%, UTS = 200 MPa). The EL was further increased to 27.3% (~+142%) after four passes of ECAP comparing to the as-extruded condition, which was mainly caused by the much more homogenized microstructure. The split basal poles with about 60 • rotations to the extruded direction (ED), the relatively coarsened grain size by static recrystallization (SRX) and post-dynamic recrystallization (PDRX) after four passes of ECAP might be responsible for the decreased strength with increasing ECAP pass. During the annealing treatment, recovery dominantly occurred at 300 • C, SRX and grain growth emerged at 350 • C and 400 • C, respectively. Meanwhile, the grain grew and hardness decreased rapidly even within 5 min for 1-pass ECAPed material at 400 • C, indicating a larger grain boundary mobility of ECAPed materials induced by higher deformation energy than the as-extruded ones.