High thermal conductive Si3N4 is a promising candidate for producing high quality ceramic substrates used in vehicles. Although a lot of researches have been carried out on the tape casting of Si3N4, they were mainly restricted in fabricating thin films with a thickness less than 200μm which are not sufficient for producing substrate. With larger thickness, tapes tend to crack during drying. In this work, slurry of α-Si3N4 with Y2O3-MgO as additives was prepared using an azeotropic mixture of methylisobutylketone/methyehtylketone/cyclohexanone as media. To obtain thick green tapes, the amount of dispersant, binder and plasticizer in the slurry were optimized, and the effects of drying temperature on the cracking behavior of the tape were studied. By controlling the composition of the slurry and the drying conditions, thick green tapes with thicknesses up to 1.8mm were successfully fabricated, and Si3N4 substrates were produced after subsequent gas-pressing sintering, which were flat and had a relative density as high as 98.2%.