Stable aqueous Al2O3 suspensions that can be flocculated by increasing temperature were prepared using tri-ammonium citrate and poly(acrylic acid) (PAA). Effects of particle volume fraction on the rheological properties of Al2O3 suspensions were investigated. The modified Dougherty-Krieger equations were used to describe the volume fraction dependence of relative viscosity for both the PAA-addition and PAA-free suspensions. An equation using percolation model was proposed to analyze the temperature and volume fraction dependence of the relative viscosity of the suspensions. The calculated results were in good agreement with the experimental data.