A new coordination number model for square-well fluid is proposed in this work. The model modifies the theory previously developed for the coordination number of square-well fluids derived on the basis of the generalized van der Waals theory by Largo and Solana. In the present work, we have calculated the co-ordination number of a square-well fluid on the basis of high temperature expansion (HTE) approach along with the modified formula of the maximum reduced density and a novel approach to derive the coordination number of hard sphere fluids. We have also performed the theoretical predictions of constant-volume excess heat capacity of the square-well fluids. It is shown that the theoretical predictions of the constant-volume excess heat capacity determined by second-order perturbation term are in much better agreement with simulation data than to the values of C-V(E)/NK obtained by other workers due to the better description of the second-order term. (c) 2006 Published by Elsevier B.V.