The phase composition and microstructure of SiB4 powder during the process chemical vapor infiltration (CVI) of SiC were analyzed by EDS,SEM,XRD. Thermodynamic calculation was carried out in order to investigate the changes of SiB4 powder. The results show that the SiB4 particles do not decompose during CVI SiC process,but they are oxidized to form SiO2and B2O3 near the external surface of the specimen. Therefore, the matrix of C/ SiC composite modified by SiB4 particles,by using CVI combining with slurry infiltration processes,is not homogeneous and compact. The uneven oxidation makes it difficult to study the oxidation mechanism of C/ SiC-SiB4composite.