Abstract: Owing to the promising application prospect of cubic magnesium oxide powders, the synthesis of magnesium oxide powders with excellent performance has great significance. This study prepared approximate nano-scale cubic magnesium oxide powders by coupling antisolvent crystallization with the reduction thermal decomposition method. The precursors were prepared with ethanol and different initial concentrations of magnesium sulfate aqueous, then the precursors were mixed with varying ratios of the weight of carbon powder to perform the reduction thermal decomposition process at 800 ℃. The results show that the particle size of precursors decreased and the microstructure of precursors became more spherical with the decreasing initial concentration of magnesium sulfate aqueous. In the reduction thermal decomposition process, the conversion rate of magnesium oxide was strongly dependent on the particle size of precursors and the additive amount of carbon powder. The conversion rate of magnesium oxide increased with the decreasing particle size of the precursor and the increasing additive amount of carbon powders. The experimental results indicate that the relatively pure cubic magnesium oxide powders could be prepared when 0.1 mol/L initial solution and the carbon mass ratio of 2 were used.