Dispersion stability and microstructural transition of colloidal silica suspensions were examined by rhe- ological measurements under either steady simple shear or os cillatory flow. Monodisperse silica particles were prepared by the so-called modified Stober method and were stabilized by either steric or electrostatic repulsive force. Depending upon the methods of stabilization, the suspension showed hard-sphere or soft-sphere response. In particular, silica suspensions exhibited hard-sphere response when the silica spheres coated with 3-(trimethoxysilyl)propyl methacry - late (MPTS; (CH3O)3Si(CH2)3OCOC(CH3)=CH2) were dispersed in a refractive-index matching solvent, tetrahydrofur- furyl alcohol. On the other hand, silica particles in aqueous media behaved like soft spheres with long-range elec- trostatic repulsive interactions when th ey were coated with steric layer of aminosilane coupling agent, N-(3-(trimethox - ysilyl)propyl)ethylenediamine ((CH3O)3Si(CH2)3NHCH2CH2NH2). In this case, the electrostatic repulsion or equiva- lently the softness of the silica spheres was controlled by th e ionic strength using a symmetric salt KCl. Both the hard- sphere and soft-sphere suspensions showed stable shear-th inning behavior without experiencing shear-induced floccula- tion. Moreover, the oscillatory shear rheology showed that the electrostatically stabilized soft-sphere suspensions underwent a microstructural transition from liquid-like to solid-li ke structure when either the particle loading increased or the ionic strength was reduced.