The influence of collector aspect ratio on the collector efficiency of upward type baffled solar air heaters has been investigated theoretically. With constant collector area, the collector efficiency increases with collector aspect ratio. This is the same results as those obtained in the previous work for flat plate solar air heaters without fins and baffles. Although the collector efficiency of baffled solar air heaters is larger than that of flat plate heaters without fins and baffles, the improvement of collector efficiency by increasing the collector aspect ratio is the reverse. (C) 2000 Elsevier Science Ltd. All rights reserved.