Gelcasting has been expected as a promising approach to prepare near-net-shaped ceramic products. The preparation of highly concentrated gelcasting slip is one of key point in this process. The rheological behavior of concentrated silicon nitride gelcasting slip was investigated with the structure effect of polymeric dispersant and the ratio of dispersant to acrylamide monomer in order to prepare highly concentrated gelcasting slip. The slip was prepared by ball milling of silicon nitride batch composed of acrylamide monomer and polymeric dispersant after premixing them by an attritor. The slip mixed with initiator will be deaired in vacuum and cast into molds, and then polymerized. The consolidated green body will be obtained by drying the gelated slip. The polymeric dispersants with different kinds of polyelectrolyte were poly(methylmetacrylic ammonium salt), poly(acrylic ammonium salt), and poly(acrylic amine salt). Before the polymerization of gelcasting slip, the viscosity vs. shear rate was measured to evaluate the rheological behavior of the slip. In case of using the polyacrylic amine salt (PAAm), the high solid loading of silicon nitride slip was obtained up to 47vol% with a low viscosity.