Continuous SiC fibre reinforced SiC matrix composites (SiC/SiC) have been studied as materials for heat resistant and nuclear applications. Thermal stability is one of the key issues for SiC/SiC composites. In this study, 3D SiC/SiC composites are fabricated via the polymer impregnation and pyrolysis (PIP) process, and then heat treated at 1400~1800癈 in an inert atmosphere. The effect of heat treatment on microstructure and mechanical properties of the composites is investigated. The results indicate that the mechanical properties of the SiC/SiC composites are improved after heat treatment at 1400癈 mainly because the mechanical properties of the matrix are greatly improved due to crystallisation. With the increasing of heat treatment temperature, the properties of the composites are conversely decreased because of severe damage of the fibres and the matrix.