Polycrystalline cesium iodide (CsI) thin films were prepared on glass substrates by thermal evaporation. The Influences of air exposure on the structure, resistivity and infrared transmittance of CsI film were investigated by scanning electron microscopy, X-ray diffraction (XRD), high resistance meter and infrared spectrophotometer (IR). It is found that the coalescence of grains occurs and the average grain size increases from 0.36 μm to 1.25 μm. The mechanism of grain growth is attributed to the diffusion of water molecules along grain boundaries and the migration of grain boundaries driven by minimization of total free energy. XRD results indicate the formation of (110/220) texture when exposed to ambient air and the relaxation of tensile stress during recrystallization. The average crystallite sizes obtained from Debye-Scherrer's formula are 25.6 nm, 28.4 nm and 45.1 nm respectively. The resistivity of the film decreases from the order of 1010 Ω·cm to 108 Ω· cm. The IR absorption bands in the ranges of 3675-3750 cm-1 and 3560-3640 cm-1 closely resemble that of free water rather than liquid water. The observed split bands are assigned to the non-hydrogen-bonded OH associated with ion-dipole bonds and dangling OH at air-water interface respectively.