Amorphous　Mg-based　hydrogen-storage　alloy　(Mg70.6Ni29.　4)90Nd10　was　prepared　by　melt-spinning.　The　phase　structures　of　the　ribbons　before　and　after　charge/discharge　cycling　were　characterized　by　high-resolution　transmission　electron　microscopy　(HRTEM)　and　X-ray　diffraction　(XRD)　analysis,　respectively.　It　was　found　that　the　amorphous　structure　began　to　crystallize　after　four　cycles　of　hydrogenation/　dehydrogenation　and　a　new　nano-crystallized　phase　NdMg2Ni9　was　detected　with　average　grain　size　in　the　range　5　nm.　The　nano-size　Mg2Ni　phases　appeared　after　6~10　cycles,　and　the　stable　Mg2Ni,　α-Mg　and　Nd2H5　phases　were　present　after　20　cycles　of　charging/discharging.　That　indicated　that　the　NdMg2Ni9　phase　formed　at　the　beginning,　also　gradually　decomposed　into　Mg2Ni,　α-Mg　and　Nd2H5　phases　as　hydrogenation/dehydrogenation　proceeded.　The　following　results　of　thermodynamics　calculation　verified　that　the　nano-crystallized　phase　NdMg2Ni9　was　present　firstly　during　the　process　of　crystallization,　because　its　free-energy　(ΔG=-15.　2789　kJ·mol-1,　T=300　K)　was　the　lowest　among　Mg-Ni　phase　(ΔG=-8.　2694　kJ·mol-1,　T=300　K)　and　Mg-Nd　phase(ΔG=-13.　29503　kJ·mol-1,　T=300　K).