The test results of fracture toughness for three wood species, such as pine, alder and birch are presented. Examination of fracture toughness is carried out using mode II fracture (shearing). Values of the stress intensity factor, K-llc, are determined for the three main anatomic directions of wood. Microstructural tests of particular wood species, performed on specimens along the three main anatomic directions of wood, are discussed. Qualitative relationships a refound to exist between the microstructure of wood a nd the obtained values of the stress intensity factor, K-llc.