We carried out classical atomistic studies of crack propagation in fully three-dimensional nanocrystalline alpha-Fe (body-centered cubic structure) to examine the influence of temperature and average grain size on the fracture mechanisms and properties. Digital samples with grain sizes ranging from 6 to 12 nm are reported at temperatures ranging from 100 K to 600 K using atomistic simulations. For all grain sizes, a combination of intragranular and intergranular fracture is observed. Mechanisms such as grain boundary accommodation, grain boundary triple junction activity, grain nucleation and grain rotation are observed to dictate the plastic deformation energy release. Intergranular fracture is shown to proceed by the coalescence of nanovoids formed at the grain boundaries ahead of the crack. The simulations also show that at an atomistic scale the fracture resistance and plastic deformation energy release mechanisms increase with increasing temperature. The observed fracture toughness increases with decreasing grain size.