Abstract: On the basis of turbulent theory and the results observed by high speed strobo-scopic microscopic photography for meso-and micro-mixing processes, a mixing mechanism was proposed and then a new mixing - precipitation model including micromixing, turbulent dispersion(meso-mixing)and backmixing(macro-mixing)was established to predict the par-ticls size distribution (PSD). By this model the values relevant to the critical points on the curves of the effects of mixing factors, i. e. ,stirrer speed and feed time on PSD were first quantitatively predicted. The results agreed well with those of experiments. This fact denotes that the model is quite reasonable, practical and capable of prediction.