The influence of the constant life diagram (CLD) formulation on the fatigue life prediction of composite materials was examined. Most commonly used and recently proposed CLDs applied on composite materials fatigue data are presented and their applicability on a number of data sets is demonstrated. Composite material fatigue data from a number of well documented databases were used. The influence of the selected CLD formulation on the fatigue life prediction of composite material under spectrum loading is assessed by its ability to accurately predict unknown S-N curves for arbitrary loading. The results revealed that the simple piecewise linear formulation compares favorably to other more sophisticated and complicated schemes. For most of the cases studied, the S-N predictions based on the piecewise linear CLD are the most accurate ones. © 2009 Elsevier Ltd. All rights reserved