Single crystal iridium exhibits anomalous deformation behaviors in contrast to other fcc-metals and its intrinsic deformation mechanism remains controversial. To investigate the deformation behaviors and underlying deformation mechanisms with respect to crystallographic orientations in single crystal iridium, molecular dynamics simulations were performed at 1 K. Bulk single crystal iridium with different loading axis orientations of [100], [110] and [111] has been considered in current study. Atomic simulation results showed that the on the stress–strain curves differed significantly between crystallographic orientations. And the mechanical properties including elastic modulus, yield stress, ultimate tensile stress and elongation more or less differed between crystallographic orientations owing to different deformation mechanisms. Under tensile loading, [100] oriented single crystal iridium deformed predominantly by dislocations glide with partial vacancies coalescence involved, while plastic deformation in [110] oriented single crystal iridium was initiated by stacking faults. Nevertheless, [111] oriented single crystal iridium underwent little plastic deformation before breaking.