Selected microstructural states of pure tungsten sheets with thicknesses of 1, 0.5, 0.2 and 0.1 mm were tested at room-temperature (RT) and 200 °C regarding their fracture properties with focus on the occurrence of an R-curve (crack growth resistance curve) behavior. Through thermomechanical rolling and the resulting increase of deformation a pronounced refinement of microstructure and strengthening of the rotated cube texture was induced with decreasing sheet thickness. The fracture experiments exhibited large variations of the fracture behavior depending on the testing temperature, material thickness and microstructure: While at RT, the 1 mm material did not exhibit R-curve behavior, it was found at 200 °C irrespective of the sample thickness. With decreasing sample thickness and grain-size the RT fracture-behavior changed and showed the onset of an extended R-curve behavior for the 0.5 mm followed by an increasing extent of this characteristic for the thinner material states. The change of the fracture behavior at RT is associated with a transition of the failure behavior from transcrystalline fracture for the thicker samples to a mixture of transcrystalline with delamination fracture to uniform delamination failure for the thinnest material. These observations suggest a continuous shift of the ductile to brittle transition temperature from 200 °C to RT and even below RT with decreasing sample thickness. The fracture toughness in terms of the maximum stress intensity, Kmax, increased at RT from approximately 20 MPam1/2 for the 1 mm samples to values up to 60 MPam1/2 for the thinnest material states, whereas at 200 °C this increase was less pronounced.