Abstract: Intergranular slow crack growth (SCG) in ceramic polycrystal is described with a cohesive zone model that mechanically simulates the reaction-rupture mechanism underlying stress and environmentally assisted failure. A 2D polycrystal is considered with cohesive surfaces inserted along the grain boundaries. The anisotropic elastic modulus and grain-to-grain misorientation are accounted for together with an initial stress state related to the processing. A minimum load threshold is shown to originate from the onset of the reaction-rupture mechanism to proceed where a minimum traction is reached locally and from the magnitude of the initial compression stresses. The cohesive model incorporates a characteristic length scale, so that size effects can be investigated. SCG is grain size dependent with the decrease of the crack velocity at a given load level and improvement of the load threshold with the grain size. Polycrystals of zirconia and alumina are both considered in the current study and a comparison between SCG resistance of alumina and zirconia is presented. This work aims at providing reliable predictions in long lasting applications of ceramics.