An experimental study concerning the frost formation and the airside performance of fin array is conducted. Visual observation indicates that the frost thickness t at the fin base is thicker than that at the fin tip. During the frost formation along the fin array, one can see an apparent boundary separating a very thin frost thickness (the region does not show appreciable frost formation) and a region with appreciable increase of frost thickness. Furthermore, frost separating boundary is not horizontal but inclined to the air flow direction. In the entrance region, the frost is apparently fewer than that at the rear region. If part of the fin surface is above the freezing point temperature but is still below the dew point temperature, one can experience that moisture condenses on the upper part of the fin while frost formed nearby the fin base. The water condensate on the upper part of the fin may fall off to wash off the porous structure of the frost to become a dot-like pattern. With a fixed frontal velocity, the heat transfer coefficient for Fs = 3 mm is slightly increased with elapsed of time. In this regard, the heat transfer coefficients for Fs = 3 mm are higher than that of Fs = 8 mm. The corresponding pressure drops of the fin array is roughly proportional to Fs0.54.