Little is known about fracture toughness of monomer-impregnated wood biocomposites, with no data for transparent wood. For building applications, toughness is important, but the anisotropic nature of wood generates complex stress distributions that need to be considered. A four-point bending fracture mechanics test is analyzed with digital image correlation (DIC) measurements of strain fields and finite element modeling. The elastic parameters of an orthotropic composite and the parameters for a cohesive zone fracture model are determined from one crack growth experiment. The effects on the fracture and deformation mechanisms from the polymer matrix and delignification on the wood substrate are investigated. The fracture toughness for tangential-longitudinal (TL) cracks was lower than expected, as explained by local peeling fracture of the wood cell wall.