We created heat exchanger louvered fin designs in which the louver pitch successively increased or decreased by 20% from the air inlet region to the redirection region. The frost behavior and thermal performance of the unequal louver pitch design were compared with those of an equal louver pitch design, under both dry and frost conditions. When unequal louver pitch design was used, frost blocking of the spaces between louvers at the front side was delayed and the thermal performance was improved by 21%, compared with the equal louver pitch design. The frost blocking ratios of the two unequal louver pitch designs were compared; the design in which the louver pitch successively decreased from the air inlet region to the redirection region provided more uniform frost growth and improved thermal performance, compared to the design in which the louver pitch successively increased. (C) 2015 Elsevier Ltd. All rights reserved.