Silicon carbide (SiC) is one of the advanced ceramics, which is widely used in industries due to its excellent mechanical properties. Understanding the relations between its microstructure and the mechanical properties is critical to adopting SiC ceramics in different applications. In this paper, a multiscale model incorporating a cohesive zone model is proposed to predict the mechanical properties of SiC ceramics. Interatomic potentials are developed using ab initio calculation to more accurately calculate the SiC behaviors in molecular dynamics modeling. The proposed multiscale model is used to predict the mechanical properties of SiC ceramics and their relations with the grain size distribution in the finite element framework. A good agreement is found between prediction results and experimental measurements. Successfully predicting its mechanical behaviors could help selection of parameters during processing of SiC ceramics under different conditions.