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Grain boundary precipitation and segregation play an important role in determining mechanical properties of Mg alloys. In the present work, we studied work focuses on the strengthening and deformation mechanism of coarse-grained (CG) and fine-grained (FG) Mg-Gd-Y-Ag-Zr-Ce alloy. The CG alloy is strengthened by means of age-strengthening with the formation of both basal plate γ′′ and prismatic plate β′ precipitates in the grain interior. While the strengthening of FC alloy is completed by intergranular alloying segregation and intragranular precipitates γ′′ and β′. The segregation of alloying elements at the grain boundary and formation of sub-micron particles can stabilize the grain boundary and suppress the intergranular deformation. Consequently, dislocations could be trapped near γ′′ and β′ precipitates in the grain interior. Unlike CG alloys, the FG alloys exhibit a heterogeneous transition from elastic to plastic deformation via the Lüders plateau. The rapid gliding dislocation multiplications and fine-grained size are necessary and sufficient conditions for the Lüders strains. Our work provides the insights on the evolution of fine-grained microstructure and helps for the design of Mg alloys with good mechanical properties. [ABSTRACT FROM AUTHOR]