Infrared emissivity of high quality indium tin oxide (ITO) film has been calcula ted based on the infrared radiation theory and thin film optical theory, the the oretical curves and the testing curves basically agree with each other. It is concluded that when the sheet resistance is less than 30Ω, the theoretical val ue of infrared emissivity of ITO films on the infrared wave band of 8μm to 14μ m will be less than 0.1. Therefore, the ITO film of practical sheet resistance less than 10Ω has good infrared stealthy capability. Physical mechanism of low infrared emissivity for ITO film is discussed, and the critical sheet resistance of low infrared emissivity, which conduce to the theoretical study and the ma nufacture of infrared stealthy ITO film, is put forward in this paper.