Oxidation tests of a three-dimensional C/SiC composites with different coatings were conducted in dry air and the effect of defects in the coatings on the oxidation behavior were investigated. There existed two kinds of preparation defects in the CVD SiC coating, including the supporting defects and the machining defects. The former were produced by supporting of the specimens and the later were produced from the matrix pores formed between the fiber bundles after machining. There existed two kinds of deposition defects in the multi-layer CVD SiC coating, including the plane-defects and the net-defects. The former were formed between the layers and the later were produced at the boundaries between the particles. Increasing the coating thickness can seal the machining defects, but it can not seal the supporting defects. Multi-deposition can not completely seal the preparation defects although it decreased remarkably the weight loss. The Si-Zr outer layer can seal completely defects in the multi-layer, but it increases the weight loss at low temperature, and furthermore has a low resistance to oxygen diffusion. The borosilicate glass can seal the coating cracks, but it nearly loses all ability of sealing defects at high temperature. The preparation defects can be completely sealed if there exist no plane-defects in the multi-layer coating. Increasing the deposition temperature and decreasing the deposition pressure will be favorable to removing the plane-defects.