This paper studies the thickness effect on the fracture energy of cementitious materials based on a local fracture energy concept. Similar to the specimen back boundary, the presence of two free surfaces in the thickness direction also influences the local fracture energy dissipation, leading to the boundary or thickness effect. A bilinear local fracture energy model originally developed to characterise the ligament or back boundary effect on the fracture energy is further developed to consider variations of the local fracture energy in the thickness direction. The proposed model is used to analyse available experimental data from the literature. The predictions from the model are in a good agreement with the experiments. (C) 2002 Elsevier Science Ltd. All rights reserved.