Neutron irradiation uniformly produces vacancies and interstitials in silcon carbide (SiC) poly crystals, and the specimen swells by 1 to 3%. Subsequent isochronal annealing leads to annihilation of the defects by the interstitial-vacancy recombination from around irradiation temperature, resulting in the shrinkage of the specimen. This shrinkage can be detected by measuring the specimen length with a conventional micrometer and its lattice parameter with an X-ray diffractometer. Furthermore, defect formation and annihilation affect the electrical resistivity and create paramagnetic centers caused by unpaired electrons. Helium atoms can be uniformly introduced into SiC utilizing the nuclear reaction of 10B(n, α)7 Li. By subsequent annealing above ∼1300°C, helium atoms with high vibration energy capture thermal vacancies to reduce the internal pressure and form bubbles at grain boundaries. The formation of helium bubbles accompanies a large volume expansion with increasing temperature, controlled by Greenwood et al.'s mechanism. The presence of helium bubbles at the grain boundaries promotes diffusional creep at lower temperatures (1300°C). Changes in physical properties by neutron irradiation are presented and discussed with respect to microstructures.