An investigation of shear yield stress is made on well-characterized alumina suspensions of different distributed particle sizes at the vicinity of the particle isoelectric point (IEP) across a wide range of volume fractions. Experimental results are compared with recently developed models [Kapur et al. (1997); Scales et al. (1997)] and structural effects on the yield stress are examined. The models predict the magnitude order of the yield stress below a volume fraction of approximately 0.42, suggesting that interparticle forces play a dominant role in determining the network strength in this concentration region. Deviations between experimental results and theoretical predictions are explained in terms of structural effects being controlled by a competition between weak particle-particle linkages and geometric resistance on the network strength. At higher volume fraction, the effect of geometric resistance on the deformation of suspensions becomes more pronounced. A number of models for the yield stress of size distributed suspensions an then proposed. Results suggest that the effect of polydispersity of particles on the yield stress of suspensions can be well characterized by a surface area average diameter and the broad size distributed suspension exhibits a higher yield stress than the narrow size distributed suspension of the same volume average diameter.