The hard sphere expansion conformal solution theory is a very useful technique to predict the thermodynamic properties of a mixture. When we apply this theory, an important problem is the method of determining the effective diameters. Hwu developed a diameter evaluation method using pure component P-V-T data represented by accurate equation of state. This method requires a very time consuming iterative process. Consequently, if it is possible to obtain an analytical equation that expresses accurate diameter values in terms of known variables, we can shorten the computation time. In addition, one can simplify the prediction of derived thermodynamic properties from a single correlation of the optimal diameters for compressibility factor computation. Hard sphere diameters for light hydrocarbons (i.e., methane, ethane, and propane) were calculated using the hard sphere expansion conformal solution theory. The diameter correlation is presented in generalized form as a reduced temperature, reduced density, and acentric factor. Thermodynamic properties for methane-propane mixtures were calculated using the diameters from this diameter equation. The calculated results showed a good agreement with experimental values. The diameters for simple fluids were calculated and compared with those values reported by Bienkowski and Chao. Other than the same trend of temperature dependence, there was no agreement between them, indicating that the optimal values to accompany a particular method of determining the attractive contributions are not closely related to an actual hard molecular core.