The effects of turbulent mixing on barium sulfate precipitation in an imperfectly mixed double jet semi batch reactor were investigated experimentally and theoretically. When two feed solutions in separate streams were fed into the semi batch reactor, the precipitation was significantly altered by the impeller speed and the feeding time. Generally, in the range of low impeller speed (below 400 rpm), the suspension was segregated vertically in the reactor and the average particle size increased with increasing impeller speed. However, in the range of high impeller speed (above 400 rpm) the suspension was homogeneously dispersed in the reactor, but the trend of the turbulent mixing effect on the precipitation was opposite to that in range of the low impeller speed. The precipitation in the semi batch reactor was controlled by particle mass transfer and micromixing of the feed streams, both of which were promoted by increasing the impeller speed. At low impeller speed the influence of the mass transfer was dominant so that the particle size increased with increasing impeller speed, but at high impeller speed it was surpassed by the influence of micromixing so that the trend was reversed because enhanced micromixing generates a large number of small particles in the reactor. To model our hypothesis for the effects of imperfect mixing on the reaction precipitation in the semi batch reactor, a micromixing-limited plug flow-ideal semi batch series reactor model was developed. The model predicts that enhanced micromixing created high supersaturation levels in the premixing region (plug flow reactor) which reduces the average particle size. The model also predicts the effect of feeding time on the precipitation in the semi batch reactor. These predictions are in excellent agreement with the experimental data. An interesting prediction of our model is that micromixing in the premixing region plays an important role in the overall reaction precipitation and its effect is greatly intensified as the turbulent mixing intensity is increased, which is opposite to our common sense.