By using high temperature optical microscopy, oxidation behaviors of poly- and single crystalline NbSi2 were observed in-situ at 1023 K. The effects of micro-cracks, porosity and grain boundary on the oxidation behavior of NbSi2 have been studied. The results indicate that new cracks initiate and extend from the pre-existing cracking areas in the arc-melted poly-crystalline specimen, leading to fragmentation after 220 min at 1023 K. However, although pores and grain boundary are the preferential oxidation site, they do not directly lead to fragmentation during oxidation, indicating that the pre-existing cracks in specimen are the key reason to pesting. The oxidation kinetics of the studied NbSi2 specimens corresponds well with the in-situ observation.