Characteristic features of floc strength were examined by theoretical and experimental studies by using clay-aluminium flocs. From the studies, the following conclusions were obtained. 1. (1) Maximum floc diameter d max under an agitation intensity is evaluated by the following equations: ▪ where, ϵ 0: the effective rate of energy dissipation [erg cm −3·s], K p : the exponent of the floc density function [−], λ 0: micro scale of the isotropic turbulence [cm], d max: the maximum diameter of flocs [cm], K,K ∗ : constants. In practice, however, the latter equation is possibly used. 2. (2) From the above equation in viscous subrange, and K p -value of the floc density function evaluated in the previous paper, d max will be changed with the rate of energy dissipation ϵ o or the rate of the agitation blades rotation Nr as the following equation: ▪ 3. (3) Relative floc binding strength can be evaluated by the following equation: ▪ where, σ: the floc binding strength, a: coefficient of the floc density function, d: diameter under an agitation intensity, Suffixes 1 and 2 denote the characteristic values of flocs 1 and 2, respectively. 4. (4) After the theory and experimental studies, characteristic features of clay-aluminium flocs were evaluated.