The relationship between the surface structures of skin layers of crosslinked aromatic polyamide composite reverse osmosis (RO) membranes and their RO performances have been studied using two surface analytical techniques : SEM and AFM. As a result, it was found that RO membranes whose skin layer surface structures were rough produced high fluxes, and an approximately Linear relationship existed between this surface roughness and RO membrane flux. Accordingly, skin layer surface unevenness of crosslinked aromatic polyamide composite RO membranes is regarded as an enlargement of the effective RO membrane area.