The keystones of the start-up of upflow anaerobic reactor with flocculent sludge as seed include the improvement of COD removal and sludge granulation. The anaerobic sludge granulation consists of two steps, namely nucleation and maturation upon nuclei. The nucleation as the starting point is of particular importance. In this paper, the nucleation of flocculent sludge as seed under low, medium and high hydrodynamic shear conditions is studied with an original quantitative method. The average sludge diameters (ASD) or nucleus ratios show satisfactory linear correlations with the operation time during the nucleation and the average augmentation rate of ASD of 0.40, 0.51 and 0.41 microm x d(-1) respectively. The nucleation under the medium shear conditions of shear rate of about 8.28 s(-1) which is corresponding to the superficial liquid and gas velocities of 2.66 and 0.24 m/h develops fastest. High hydrodynamic shear conditions enhance the improvement of COD removal of reactor. In this study the increase rate of ASD and the improvement rate of COD removal to 92% of sludge show consistent trend.