The permeat flux in a tubular-membrane can be improved by inserted concentrically with a solid-rod of linearly varied rod-radius and fixed rod volume. The performance in the device with rod-radius increased overcomes that with rod-radius decreased. The performance further improves as the rod-radius increases more rapidly. Experimental results show that above 90% improvement in permeated flux was achieved, based on the device without a solid-rod. (C) 2004 Elsevier B.V. All rights reserved.