Large scale molecular dynamics (MD) simulations are carried out to investigate the failure response of nanocrystalline Mg using the EAM potential under conditions of uniaxial tensile stress and uniaxial tensile strain loading. The MD simulations are carried out at a strain rate of 109s-1 for grain sizes in the range of 10 nm to 30 nm. The effect of grain size on the strength of the metal is investigated and the critical grain size for transition to inverse Hall-Petch regime is identified.