The oxidation behaviors of ZrB2- 30 vol% SiC composites were investigated at 1500°C in air and under reducing conditions with oxygen partial pressures of 104 and 10−8 Pa, respectively. The oxidation of ZrB2 and SiC were analyzed using transmission electron microscopy (TEM). Due to kinetic difference of oxidation behavior, the three layers (surface silica-rich layer, oxide layer, and unreacted layer) were observed over a wide area of specimen in air, while the two layers (oxide layer, and unreacted layer) were observed over a narrow area in specimen under reducing condition. In oxide layer, the ZrB2 was oxidized to ZrO2 accompanied by division into small grains and the shape was also changed from faceted to round. This layer also consisted of amorphous SiO2 with residual SiC and found dispersed in TEM. Based on TEM analysis of ZrB2–SiC composites tested under air and low oxygen partial pressure, the ZrB2 begins to oxidize preferentially and the SiC remained without any changes at the interface between oxidized layer and unreacted layer.