Recently, textured microstructure has received particular interest in the processing of advanced Si3N4 ceramics because of significant improvement in the mechanical properties and thermal conductivity. This work will report a highly textured β-Si3N4 ceramic by aqueous slip casting in a magnetic field and subsequent pressureless sintering, using commercial α-Si3N4 raw powder and a mixture of Y2O3 and Al2O3 as sintering aids. To obtain the well-dispersed Si3N4- Y2O3-Al2O3 slurries, polyethylenimine (PEI) was chosen as a dispersant. Effects of the sintering aids, PEI amount, pH and stirring time on the stability of the Si3N4 slurries were studied. It is shown that PEI is an effective dispersant for stabilizing the Si3N4-Y2O3-Al2O3 slurries that does not show a time-dependent behavior at an optimum pH ≈ 10, compared to the case in the absence of PEI. Using the 30 vol% Si3N4-Y2O3-Al2O3 slurries stabilized with 1.5 dwb% PEI at pH 10, the highly textured β-Si3N4 with 97 % relative density could be obtained by slip casting in a magnetic field of 12 T and subsequent sintering at 1800 oC for 1 h. The textured microstructure is featured by the alignment of c-axis of β-Si3N4 crystals perpendicular to the magnetic field, and the Lotgering orientation factor, f, is determined to be 0.8.