Silicon carbide fibre reinforced silicon carbide matrix composite (SiC/SiC) is a key structural material for fusion reactors to allow high temperature operation of liquid lithium breeder blankets. This research investigated the corrosion behaviour of two SiC/SiC composites (using Tyranno SA3 and SA4 fibres) immersed in static liquid lithium at 600 °C for 100 h. Utilizing immersion tests and advanced microstructural analysis, the study reveals considerable corrosion in SiC/SiC composites, particularly in local areas enriched with residual carbon or oxygen. The interface carbon layer, while enhancing mechanical properties, induces preferential sites for corrosion cracking, diminishing the material's corrosion resistance. This study offers critical insights into the corrosive interaction between SiC/SiC composites and liquid lithium, highlighting the need for manufacturing process optimization and exploration of alternative interface layers, protective coatings, or avoiding contact between SiC/SiC and molten lithium in breeder blanket system.