The aggregation and breakup of particles in a Couette flow system were studied for different values of the shear rate (G) and the volume fraction of particles phi(0). After a certain time, an initial distribution of 2-mu m-dia. latex particles achieves the steady state, reflecting the balance between coagulation and fragmentation forces. Higher G values shift the steady state to smaller aggregate sizes. For a fixed value of G, when phi(0) increases, the steady-state size increases if the flow is laminar and decreases if the flow is turbulent The population balance model proposed describes simultaneous coagulation and fragmentation during shear flocculation. A power dependence between the breakage rare coefficient and the shear rate was found. The model is adjusted by wing two parameters : the probability of success for collisions a and the effective breakup coefficient for shear fragmentation B＇(eff) depends on both G and phi(0), while alpha is nearly constant.