Silicon carbide is one of the most important semiconductors with wide bandgaps and various applications including power electronics, nuclear fuel particles, hostile-environment electronics, and blue light emitting diodes. We investigate the nonlinear mechanical properties of a proposed graphene-like planar hexagonal silicon carbide (g-SiC) monolayer using first-principles calculations. The strength of g-SiC is about half that of graphene. The ultimate strain of g-SiC is 0.2, 0.25, and 0.19, in the direction of in armchair, zigzag, and biaxial, respectively. The Poisson’s ratio is 1.75 times of that of graphene. In the nonlinear elasticity regime, we obtain the high order elastic constants up to fifth order. The stiffness monotonically increases with pressure, has the same trend as that of second order elastic constants but opposes to that of Poisson’s ratio. There is a minimum at −4 GPa in the speed-pressure curve of compressive sound wave, different from the monotonic increment of shear waves. These theoretical mechanical properties provide elasticity limits for various applications of g-SiC [1].