The hardness of 4H-SiC, which was high-temperature (500 K) helium-implanted to fluences of 3×1016 ions cm-2 and subsequently thermally annealed at the temperature ranging from 773 to 1273 K, was studied by nanoindentation. It is found that the hardness of the implanted 4H-SiC increases at the first, then decreases, and then increases again with increasing annealing tempeature in the temperature range of 500—1273 K, and significant increase in hardness is observed at 773 K. The behavior is ascribed to the changes of the density, length, and tangling of the covalent Si—C bond through the recombination of point defects, clustering of He-vacancy, and growth of helium bubbles during the thermal annealing.