This study presents the development of a novel rheodielectric sensing technique, based on dielectrostriction measurement, for online injection molding process monitoring. Dielectrostriction, defined as a variation of dielectric properties of material under deformation, detects evolution of molecular orientation during injection molding and enables online shear stress measurement. The dielectrostriction effect resembles the well-known birefringence phenomenon. As birefringence in polymers is described by the stress-optical relationship (Fuller, Optical Rheometry of Complex Fluids, Oxford University Press, New York (1995); Janeschitz-Kriegi, Polymer Melt Rheology and Flow Birefringence, Springer, Berlin (1983); Saiz and Rainde, Dipole Moments and Birefringence of Polymers, Prentice-Hall, New Jersey (1992)), a stress-dielectric relationship exists for and is applicable to dielectrostriction. In addition, dielectrostriction measurements can be performed on both transparent and opaque materials with a much simpler data acquisition technique. To demonstrate the feasibility of the dieletrostriction measurement, a planar capacitor sensor rosette has been developed and attached to the surface of an injection mold to obtain the dielectrostriction signal under various processing conditions. The calculated shear stresses of polymer melts based on dielectrostriction signals were validated by agreement with simulation predictions. POLYM. ENG. SCI., 50:61-68, 2010. (C) 2009 Society of Plastics Engineers