The oxygen free Si-C fibre (Hi-Nicalon) consists of beta-SiC nanocrystals (approximate to 5 nm) and stacked carbon layers of 2-3 nm in extension, in the form of carbon network along the fibre. This microstructure gives rise to a high density, tensile strength, stiffness and electrical conductivity. With respect to a Si-C-O fibre (Nicalon NL202), the Si-C fibres have a much greater thermal stability owing to the absence of the unstable SiOxCy phase. Despite its high chemical stability, it is nevertheless subject to a slight structural evolution at high temperatures of both SiC and free carbon phases, beginning at pyrolysis temperatures in the range 1200-1400 degrees C and improving with increasing pyrolysis temperature and annealing time. A moderate superficial decomposition is also observed beyond 1400 degrees C, in the form of a carbon enriched layer whose thickness increases as the pyrolysis temperature and annealing time are raised. The strength reduction at ambient for pyrolysis temperatures below 1600 degrees C could be caused by SiC coarsening or superficial degradation. Si-C fibres have a good oxidation resistance up to 1400 degrees C, due to the formation of a protective silica layer.