The expression of average shell-side mass transfer coefficient (MTC) in hollow fiber modules is theoretically deduced in this paper. This expression considers the effects of both the polydispersity of hollow fiber outer radii and the randomicity of hollow fiber distribution on the shell-side mass transfer performance for the first time. In the expression, Gaussian function is used to model the polydisperse hollow fiber outer radii, Voronoi tessellation method is used to model the random hollow fiber distribution, and the two distributions are assumed to be independent. Then, the effect of polydisperse hollow fiber outer radii on average shell-side MTC is studied in detail. (c) 2006 Elsevier B.V. All rights reserved.