The effect of the microscopic structure and the moisture content (MC) of wood on its fracture behaviour has been investigated. Green and air-dried spruce (Picea abies [L.] Karst.) and birch (Betula pendula Roth.) wood were subjected to pure mode I loading in the radial- tangential (RT) crack propagation system. Tests were carried out in situ in an environmental scanning electron microscope to observe crack propagation at the cellular level. Crack-tip displacement fields were computed by digital image correlation, and crack propagation was observed from the images captured during testing. Both the MC and the microscopic structure were found to affect the fracture process. In the air-dried birch and spruce, only microcracking caused large displacements ahead of the crack-tip. In spruce, the microcracking zone was larger than in birch. In green birch and spruce, microcracking was less evident than in the air-dried specimens, and in some cases, there were notable deformations in a few cells ahead of the crack-tip before crack extension. Microcracking is considered to be the main toughening mechanism in spruce and birch in the RT crack propagation system.