Abstract: Micromixing of reactant is the controlling step in the reaction precipitation process because chemical reaction of the formation of sparingly soluble materials is generally very fast, which would lead to uneven supersaturation distribution at the macro-level. This results in wider crystal size distribution. In order to eliminate the effect of micromixing on crystal size distribution, micromixing must be complete prior to nucleation and growth of crystal to acheve uniformity of supersaturation distribution. A new method for preparing monodispersed nanopowder is proposed. In this method, the effect of reactant micromixing on precipitation is effectively resolved in terms of the supersaturation metastable zone principle that there is a metastable zone for supersaturation and its width can be altered. The nanopowder of strontium sulfate was prepared by using this method. The morphology of crystal was observed and the particle size and particle size distribution were measured by TEM and multi-angle sub-micron particle size analyzer. The phase of matter and purity were characterized by X-ray powder diffraction. The result showed that particle size distribution of the nanopowder prepared with this method was very narrow and powder morphology was identical. In terms of the phenomenological level, the mechanism of the action of EDTA and alcohol in the reaction system was assumed.