The purpose of this study is to investigate the effects of repeated transverse rib roughness on forced convection evaporation in a narrow vertical channel. Five channels which have the same cross section measuring 2mm by 50mm (aspect ratio 25) and have different rib heights (e=0.6 and 1.0) and rib spacings (P=7, 10, 15, 20mm) were investigated. Experiments were conducted using R12 as a working fluid for vertical upward flow at mass velocities between 50 and 130kg/m2s and mass qualities between 0.2 and 0.8. Comparison of these data indicates that the heat transfer coefficient has a tendency to increase with reducing rib spacing and to decrease with reducing rib height. The highest heat transfer coefficient, which is obtained by the channel with P=7mm and e=1mm, is about three times higher than the smooth channel. This ribbed channel was also found to be best in these five channels under the same pumping power condition.