The effect of large thickness-reduction on microstructure evolution and tensile properties of Mg-9Al-1 Zn alloy (AZ91) processed by hard-plate rolling (HPR) was investigated.Increasing rolling reduction from 55 ％ to 85 ％ increases the volume fraction and refines average size of fine grains (＜ 3 μm,FGs),leading to an optimized bimodal-grained structure consisting of coarse grains (CGs) uniformly embedded in FG regions.The sample with 85 ％ reduction exhibits the highest yield strength of ～314 MPa,ultimate tensile strength of ～381 MPa and elongation of ～11 ％.The high strength is primarily due to the contribution of grain boundaries (GBs) strengthening by FGs (accounting for ～65 ％ of strength),meanwhile the improved ductility originates from the optimized bimodal-grained structure and weakened basal texture that favor a higher ductility.The present findings successfully overcome the trade-off dilemma that the large-reduction rolling processing on Mg alloys usually enhances strength at expense of ductility.In addition,the intensified heterogeneous deformation and favorable formation ofa bimodal-grained microstructure during large-reduction HPR was addressed by tracing microstructure evolution details in grains of interest via quasi-in-situ electron back scattering diffraction (EBSD).The present study can be instructive for further designing novel Mg alloys by tailoring bimodal-grained structures for superior combination of mechanical properties.