Frost deposition and its growth occur on the cold surface of the heat exchangers by being exposed to humid air, which cause serious problems on the heat transfer performance due to the thermal resistance between the cooling surface and the ambient air. The structure of frost is porous consisting of ice crystal and air, which makes the thermal conductivity of frost remarkably low. In this study, the microstructure of the frost layer is measured by using the X-ray CT at SPring-8 BL20B2 beam-line. Frost is formed on the cooling surface of 6 mm in diameter. 600 frost-layer projection images are taken by the X-ray radiography with rotation of the cooling surface through 180 degrees in 45 seconds. The reconstructed three-dimensional image clearly showed the microstructure of the frost layer, which consisted of three regions; the ice droplets region, the columnar crystals region and the dendrite crystals region for the 10 min frosting duration. The ice droplets were formed on the cooling surface with about 80-150 m in height. The columnar crystals grew linearly from the surface of the ice droplets up to 400 m height. At higher than 400 m, the frost crystals branched and changed its structure to the dendrite crystals. For the 20 min frosting duration, the upper layer of the frost is covered by the laterally growing crystals. The frost density profile was directly estimated from the three-dimensional frost structure. The frost density showed the constant about 260 kg/m 3 at the columnar crystal region and it gradually decreased in the dendrite crystal region.

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