The present study demonstrates an analysis analytically to determine the performances and optimum design of wet annular fin assemblies of the trapezoidal profile. The Frobenius power series method is adopted as an analytical tool to solve the governing differential equation of the above type of wet fin assemblies. The performance parameters, namely, the surface efficiency and augmentation factor are determined. The present model has also ability to predict the performances of a wet fin assembly of triangular fin geometry for the selection of the very small value of the tip thickness. As the present study establishes an analytical model, it can be easily employed in determination of an optimum design condition. Both the performance and optimization study have been made by two approaches of the analysis described based on the handling of the psychrometric properties at the tip as a function of temperatures. Finally, it can be highlighted that the performances and optimum conditions of a wet fin assembly are not only dependent upon the psychrometric properties of air but also dependent upon the approach selected for calculating the energy transferred by the mechanism of mass transfer. (C) 2011 Elsevier Ltd. All rights reserved.