The present study focuses on the cracking of tropical species of the Gabonese forest such as Milicia excelsa (Iroko) and Pterocarpus soyauxii (Padouk). The critical energy release rate is computed with a modified Mixed Mode Crack Growth (MMCG) specimen. An electrome-chanical testing machine and an Arcan fixture are used in order to apply the load in opening mode (mode I) to the specimen. The force-displacement curves, the crack growth process as well as the opening crack are deduced from images of the specimen taken during the tests. These images are processed by the grid method. The energy release rate is estimated by the compliance method. The results are given for the two wood species and two different specimen thicknesses. We conclude that the thickness has an influence on the energy release rate which is estimated.