We have examined the evolution of Raman spectra of SiC fibres through structural and compositional transformations caused by heat treatment. The SiC fibre was made from polycarbosilane. Raman spectra of the SiC fibre indicate that it consists of (i) amorphous or microcrystalline SiC, (ii) carbon microcrystals, and (iii) silicon oxide. The amount of microcrystalline carbon in the fibre increases with heat treatment temperature up to 1400° C, and it decreases abruptly in those fibres heat treated above 1500° C. The tensile strength of the fibre drops virtually to zero after the heat treatment at 1500° C. Carbon microcrystals are precipitated from the Si-C random network with excess carbon, and they are distributed uniformly in the fibre. These carbon particles suppress the growth of SiC crystals. It is shown that the carbon microcrystals play an important role in maintaining the high mechanical strength of the SiC fibre.