Abstract: Plastic melt is pressed into cavity in injection molding. At the end of filling stage, the plastic melt is compacted gradually and the compressibility can’t be neglected. The PSPG (Pressure-Stabilizing/Petrov-Galerkin) method based PG (Petrov-Galerkin) theory can be employed to suppress the spurious numerical oscillation due to the non-matching of interpolation functions for velocity and pressure when solving the momentum governing equation. In addition, the SUPG (Streamline-Upwind/Petrov-Galerkin)/GGLS(Galerkin gradient least-squares) method can be applied to obtain a stable numerical solution of the energy governing equation with dominated convection and little heat conduction. Therefore the stabilized finite element formulations were developed to simulate the filling stage with the effect of melt compressibility using the PSPG and SUPG/GGLS methods. Numerical examples show that melt compressibility can be neglected for most time of filling stage, but at the end of filling, compressibility leads to less increase of injection pressure and more authentic flow balance phenomenon than incompressibility.