The present study investigates hydrodynamic factors which affect the growth of maneb particles. Manganese ethylenebis, a crystalline product known as maneb, is an intermediate fungicide which is made through the reaction between ammonium ethylenebis and manganese sulfate. Crystal size distribution of maneb is under the influence of crystallizer hydrodynamics and is an effective parameter of product purity, ease of filtration, and health considerations. The present study investigates the effects of mixing intensity, location of feed entrance, use of draft tube and the now direction in the tube on the particle size of product maneb. Experiments were carried out in a 7.5 L-jacketed reactor equipped with a 45 degrees pitched 4-blade impeller, and removable draft tube and baffles. The results of the experiments showed that the mean size of particles was highly influenced by mixing intensity and had an optimum value as the mixing intensity changed. The results also suggested that the best location of feed entrance in order to produce large particles was underneath the impeller. The use of a draft tube when the now direction in the tube was downward resulted in a larger mean size of product. Furthermore, based upon the Lagrangian mechanistic model both mesomixing and micromixing mechanisms were proved effective in the reactive crystallization of maneb.