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The microstructure evolution and phase transition of as-cast AZ91 Mg alloy powders during HDDR processing were investigated. The effects of temperature, hydrogen pressure and process time on the hydrogenation and dehydrogenation were also analyzed. The whole process strongly depended on temperature. Disproportionation reaction was activated at higher temperature than hydrogenation temperature and was very difficult to complete. The appropriate higher hydrogen pressure had a positive effect on hydrogenation-disproportionation and grain refinement, however excessive pressure had opposite effect. An optimized HDDR technique for getting the nanocrystalline AZ91 Mg alloys powder is considered as follows: hydriding at 450 °C for 12 h under 4 MPa and then dehydriding at 350 °C for 4 h under 3.5E-3Pa. As a result, the average grain size of AZ91 alloy powders was refined from 100 to 300 μm to about 20 nm.The grain refinement sustained throughout the whole HDDR process, but remarkably occured in dehydrogenation stage instead of in disproportionation and recombination stages. A plate model was suggested to explain the grain refinement mechanism:hydriding cracking and dehydring craking.