An experimental study concerning the frost formation and the airside performance of a fin array is reported. Visual observation indicates that the frost thickness at the fin base is thicker than that at the tip. During the period of frost formation along the fin array, one can see an apparent boundary separating a region of very thin frost thickness (the region does not show appreciable frost formation) and a region with appreciable frost thickness. Furthermore, the frost-separating boundary is not horizontal but inclined toward the airflow direction. In the entrance region, the frost formation is apparently less pronounced 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, the moist air condenses on the upper part of the fin while frost is formed nearby the fin base. The water condensate on the upper part of the fin array may fall off to wash away the porous structure to become a dot-like pattern. With a fixed frontal velocity, the heat transfer coefficient for a fin spacing of 3 mm is slightly increased with the elapse of time and is higher than that of 8 mm fin spacing. The frost thickness increases with time but decreases with the frontal velocity.