The effect of surface modification of Si3N4 particle on the colloidal behavior and the rheological properties of aqueous Si3N4 suspensions under steady and oscillatory conditions are investigated in detail. Due to the decrease of the oxidizing level, the isoelectric point (IEP) of the modified particle shifts to basic region gently. Attempts have been made to apply rheological models to the suspensions with various solid volume fraction (phi). For the as-received suspensions, the Sisco model provides the best fit in the range of phi less than or equal to 0.30 while the Casson model in 0.35 less than or equal to phi less than or equal to 0.45. The shear behavior of modified suspensions fits to Sisco model in the range of phi less than or equal to 0.40 and Casson model in 0.45 less than or equal to phi less than or equal to 0.54. The rheological behavior of modified suspensions is improved efficiently. The critical strain decreases and the linear viscoelastic regime narrows continuously with increasing solid concentration. For the modified suspensions, the linear viscoelastic regime broadens and the corresponding elastic modulus decreases sharply. With increasing solid concentration, the characteristic frequency shifts toward lower frequencies and the suspension transforms from more viscous to more elastic.