In this study, we report the grain boundary driven mechanical behavior of 2 polycrystalline ultra-high-temperature ceramics (UHTCs), zirconium diboride (ZrB2) and zirconium carbide (ZrC) with zirconium diboride (ZrC-ZrB2). These nanocomposites were investigated using large-scale molecular dynamics simulations. First, the atomistic models of the polycrystalline ZrB2 and ZrC-ZrB2 nanocomposites were subjected to tensile loading to determine their elastic constants and tensile strengths. It was found that the presence of nanoparticles imparts an insignificant effect on the mechanical properties of ZrB2. It has also been observed that the failure mechanisms of both the ZrB2 and ZrC-ZrB2 nanocomposite are driven by grain boundary deformation. At any instant during the applied load transfer, local tensile stress distribution data indicate that atomic stress becomes much higher near the grain boundaries compared to other locations. The authors performed additional sets of simulations to obtain tensile and shear properties of grain boundary material. When these properties were compared with the adjacent single crystal and overall polycrystalline material properties, it was found that the shear strength and stiffness of the grain boundary materials are significantly lower than the single crystal or polycrystal ZrB2. It is believed that the overall deformation and failure properties of ZrB2 and its composite are controlled by the properties of grain boundary. Hence, the addition of nanoparticles played an insignificant role on the mechanical properties of ZrB2.