Deformation mechanisms in nanotwinned face-centered cubic (fcc) materials have been extensively studied due to the successful fabrication of nanotwinned fcc materials and the advance in experimental techniques and atomistic simulations. However, less attention has been paid to nanotwinned body-centered cubic (bcc) materials despite that deformation twinning has been widely observed in bcc materials both in experiments and computer simulations. Here we investigate the mechanical behaviour of nanotwinned Fe with various twin spacing under tensile deformation as a function of the inclination angle between the twin boundaries (TBs) and the loading direction using large scale molecular dynamics simulations. Our simulations reveal that the twin orientation determines the deformation mechanisms of nanotwinned Fe. When the TBs are parallel or inclined by an angle smaller than 20° to the loading direction, the samples fracture in an almost brittle manner. When the TBs are inclined by a medium angle between 20° and 70°, TB migration takes over the role of the main deformation mechanism and two possible pathways accounting for the disappearance of TBs, namely twinning or detwinning are distinguished. When the TBs are inclined by an angle larger than 70° or nearly perpendicular to the loading axis, plastic deformation is dominated by abundant slip-twin interactions. The dynamic transition in deformation mechanisms is discussed based on Schmid factor analysis and generalised planar fault energy. Moreover, we systematically summarise the plausible slip-twin interactions in bcc materials and determine the energy barriers according the Frank rule. The dislocation reactions at the TBs are compared with experimental observations.