BN of two levels of crystallinity degrees were deposited by BCl3 and NH3 on the surface of continuous SiC tows. Water-oxygen corrosion of the BN coatings was carried out at 550~1000℃, with a gas content of H2O 14%/O2 8%/N2 78% and flow rate of 6cm/min. For corrosion at 950℃ or higher, highly crystallized BN is almost run out after 0.5h corrosion; 10%, 37%, and 94% of BN is run out after corroded at 700, 800, and 900℃ for 0.5h separately. Poorly crystallized BN could be corroded comparably at temperatures ~150℃ lower than highly crystallized BN. Melt infiltrated SiC/SiC with BN interphase layer of dual crystallinities was designed and fabricated. Static fatigue test was carried out in the water-oxygen corrosion environment at 1300℃ for more than 100h until fracture. BN at the fracture surface without any pull-out effect was totally oxidized and vaporized regardless of the degrees of crystallinity. Two millimeters away from the fracture point, the inner highly crystallized BN was slightly oxidized while the outer poorly crystallized BN was partially oxidized and re-crystallized. Due to the protection of the SiC matrix, the water-oxygen corrosion rate of BN interphase layer in the composites was significantly slower than that of naked BN coatings on the surface of fibers. BN of different crystallinity degrees exhibits different oxidation behaviors, and highly crystallized BN could be protected by poorly crystallized BN.