A method for producing nano-scale theta (.theta.)-phase alumina microparticles is disclosed. The nano-scale .theta.-phase alumina microparticles are uniform in particle size and highly phase-pure. They are obtained by controlling the ratio of boehmite mixed with the .theta.-phase alumina initial powders, followed by at least one phase transformation. Therefore, the nano-scale .theta.-phase alumina microparticles produced by the present method have more uniform particle size and highly purer phase. As such for the production of nano-scale .theta.-phase alumina microparticles, the present method saves more process time and cost, and it provides an advantage such as the clean production.