In the present work,the mechanical properties of bulk nanocrystalline(NC) bcc Fe under tensile deformation have been studied by molecular dynamics(MD) simulations.Average flow stress was found to decrease with grain refinement below 13.54 nm,indicating a breakdown in the Hall-Petch relation.A change from grain boundary(GB) mediated dislocation activities to GB activities may be a possible explanation of the breakdown in the Hall-Petch relation.The results also indicate that the average flow stress increases with increasing strain rates and decreasing temperatures.Stress induced phase transformations were observed during the tensile deformation of NC Fe,and such phase transformations were found to be reversible with respect to the applied stress.The maximum fraction of the cp atoms was also found to increase with increasing applied stress.Significant phase transformation occurred in the stacking fault zone due to dislocation activities for large grain size(13.54 nm),while significant phase transformation occurred in the GBs due to GB activities for small grain size(3.39 nm).At deformation temperature of 900 K and above,no apparent phase transformation occurred because all atoms at GBs and grain interior could easily rearrange their position by thermal activation to form local vacancies/disordered structures rather than ordered close packed(cp) structures. In the present work, the mechanical properties of bulk nanocrystalline (NC) bcc Fe under tensile deformation have been studied by molecular dynamics (MD) simulations. Average flow stress was found to decrease with grain refinement below 13.54 nm, indicating a breakdown in the Hall -Petch relation.A change from grain boundary (GB) mediated dislocation activities to GB activities may be a possible explanation of the breakdown in the Hall-Petch relation. Results also indicate that the average flow stress increases with increasing strain rates and- . Stress induced phase transformations were observed during the tensile deformation of NC Fe, and such phase transformations were found to be reversible with respect to the applied stress. The maximum fraction of the cp atoms was also found to increase with increasing applied stress. of occurred occurred in the stacking fault zone due to dislocation activities for large grain size (13.54 nm), while significant ph ase transformation occurred in the GBs due to GB activities for small grain size (3.39 nm). A deformation temperature of 900 K and above, no apparent phase transformation occurred because all atoms at GBs and grain interior could easily rearrange their position by thermal activation to form local vacancies / disordered structures rather than ordered close packed (cp) structures.