Abstract: The efffects of mixing characteristics and chemical characteristics on the precipitation process conducted in a semi-batch stirred tank were investigated and analysed. It was found that there existed critical points of stirrer speed Nc, viscosity μc and feed time tfc, at which the mean size and the variance of particle size distribution (PSD) were minimal. This fact could be attributed to the interaction of micromixing and macromixing and was vital to industries such as the manufacturing of ultra-fine particles and catalysts. Experimental results also showed that the feed location possessed great effects on PSD and critical stirring speed Nc. Generally, smaller values of Nc would be achieved when the feed location was put nearer to the impeller.