The influences of helium on microstructural development and dimensional stability in high purity β-SiC after Si²⁺-ion irradiation with and without He⁺-ion injection at high temperature were studied. The microstructural observations of β-SiC irradiated up to 10 dpa at irradiation temperatures of 1073, 1273, and 1673 K were performed by transmission electron microscopy, respectively. ‘Black spot’ defects and dislocation loops were observed densely in all irradiated β-SiC. Small cavities were formed at grain interior of β-SiC above 1273 K. Helium increased the number density of cavities, but helium dose not effect on the cavity size. Swelling in β-SiC irradiated up to 3 dpa at 1273 K was measured by precision-surface profilometry. The influences of damage rate (dpa/s) and helium on the swelling were studied. The swelling values were saturated above 1 dpa after single-ion and dual-ion irradiation under higher dpa/s condition (1.0×10⁻³ dpa/s). In lower dpa/s case (5.0×10⁻⁵ dpa/s), the swelling was also saturated after single-ion irradiation, but the saturated swelling value was approximately half of the higher dpa/s case. On the contrary, the swelling value of β-SiC irradiated with dual-ion under the lower dpa/s condition increased at 3 dpa without saturation. Small cavities observed in this specimen, which were formed on {111} family planes, may cause the enhanced swelling at 3 dpa.