Abstract <p>Grain refinement typically strengthens metals (Hall-Petch effect), but at the nanoscale, strength diminishes due to changes in dislocation motion mechanisms (inverse Hall-Petch effect). However, this phenomenon is not well understood in ceramics, primarily due to restricted dislocation movement and interference from surface polishing in strength tests. In this study, we developed a dual-scale finite element model based on the crack growth resistance curve (R-curve) to eliminate surface polishing effects and investigate the impact of grain size and grain boundary fracture energy on the strength of polycrystalline ceramics. This is the first study to simulate the Hall-Petch to inverse Hall-Petch behavior at the grain scale using fracture mechanics. Our results demonstrate that grain refinement leads to a more uniform stress distribution at grain boundaries, resulting in Hall-Petch behavior, while further refinement reduces the elastic modulus, leading to inverse Hall-Petch behavior. Additionally, the proposed method for estimating theoretical strength via the R-curve cannot only be used in simulations but also improve traditional testing methods, effectively addressing the challenges in measuring the theoretical strength of large-grained samples. This work introduces a novel approach for evaluating the theoretical strength of ceramic materials, providing significant guidance for the design and fabrication of high strength and durable structural ceramics.</p> Graphical Abstract <p></p>