This paper presents the experimental results of natural convection in a rectangular cross-sectional vertical channel. Along the channel fins connected to both plates are placed, periodically. The channel walls are maintained at uniform heat flux. Rayleight number and Nusselt number are obtained based on channel width. The ratio of the channel length to the channel width, L/b, is 66. The experiments are performed for modified Rayleigh number, (b/L)Ra, ranges from 20 to 90. Results shows that Nusselt numbers for finned channel are less than those of the smooth channel. (C) 1999 Elsevier Science Ltd.