Deformation mechanism maps at 0-883 K and shear strain rate of 10-10-10+6 s-1 were built from available rate equations for deformation mechanisms in pure magnesium or magnesium alloys. It can be found that the grain size has little effect on the fields of plasticity and phonon or electron drag, though it has important influence on the fields of power-law creep, diffusion creep, and Harper-Dorn creep in the maps within the present range of temperature, strain rate, and grain size. A larger grain size is helpful to increase the field range of power-law creep but decrease that of diffusion creep when the grain size is smaller than ~204 μm. Harper-Dorn creep dominates the deformation competed to diffusion creep in the grain size range of ~204-255 μm. The maps include only plasticity, phonon or electron drag, and power-law creep when the grain size is higher than ~255 μm, then the grain size has little influence on the maps. Comparison between the reported data for the Mg-Gd-Y alloys and the maps built from available rate equations, it can be conclude that the maps are an effective tool to predict or achieve a comprehensive understanding of the deformation behavior of the Mg-Gd-Y alloys and to classify systematically their discrepancies in the deformation mechanism. However, differences exist in the deformation mechanisms of the alloys observed by the reported data and that predicted by the maps. Therefore, refinement of the maps from the viewpoint of mechanical twining, DRX, and adiabatic shear are necessary.