Purposes of this study are unsteady evaluation of thermal performance of heat exchanger with frosting and decision of optimal defrosting cycle. Thermal performance characteristics were examined. The density distribution in the height direction of the frost layer on the flat plate exists under the forced convection. The structure of the frost can't be regarded as homogeneous. The friction coefficients can be arranged by Reynolds number and the equivalent diameter ratio within ±12%.Dependence of Reynolds number is different on the experiment condition, but there are no relations in the experiment condition, and equivalent diameter ratios are almost arranged by the constant value. As for the time variation of the mass and heat transfer, reverse tendency is shown, and analogy isn't concluded. The reason is that: 1) The leading edge becomes the quadrant-shaped projection caused by the frost growth, that is made to enhance heat transfer. 2) Due to the temperature rise and partial melting of the leading edge, the infiltration of the water vapor into frost is blocked, and mass transfer doesn't increase.