In spent fuel pool （SFP） under loss-of-cooling or loss-of-coolant severe accident condition, the spent fuels would be exposed to air and heated by their own residual decay heat. Integrity of fuel cladding tube is crucial for SFP safety therefore study on oxidation of fuel cladding tube in air at high temperature is thought important. Zircaloy-2 （Zry2） and Zircaloy-4 （Zry4） were used for thermogravimetric analyses （TGA） in different temperatures in air at different flow rates to evaluate oxidation behavior. Oxidation rate increased with testing temperature. In a range of flow rate of air which is predictable in spent fuel lack during a hypothetical SFP accident, influence of flow rate was not clearly observed below 950°C for the Zry2, or below 1050°C for the Zry4. In higher temperature, oxidation rate was higher in high flow rate condition, and this trend was seen clearer when temperature increased.Oxide layers were carefully examined after the TGA analyses and compared with mass gain data to investigate detail of oxidation process in air. It was revealed that the mass gain data in pre-breakaway regime reflects growth of dense oxide film on specimen surface, meanwhile in post-breakaway regime, it reflects growth of porous oxide layer beneath fracture of the dense oxide film.