M7C3 often exhibits stacking faults (SFs). However, the influence of SFs on carbide properties are rarely studied. To explore effects of SFs, the carbide hardness, elastic modulus, and distribution of hardness and elastic modulus in the grain were analyzed using nanoindentation technology. The SFs structure of M7C3 carbides was analyzed using transmission electron microscopy (TEM). First-principles calculations revealed the influence mechanism of SFs on carbide hardness. The results showed that high-density SFs improved the carbide hardness by approximately 2 GPa. Uneven distribution of SFs leads to uneven hardness and elastic modulus of M7C3 carbides. The SFs disrupt the ordered arrangement of C-Cr bonds, which reduces the anisotropy of Cr7C3. The change in carbide strength was not due to the change in bond strength. Instead, the lattice distortion caused by the change in the C-Cr bond angle in Cr7C3-SFs increased the deformation resistance of the crystal and improved the hardness of Cr7C3.