In recent years, nano-crystalline materials have attracted many researchers’ attention, but the fracture mechanism has not been fully clarified. In a molecular dynamics (MD) simulation, grain size and crystal orientation can be chosen, and their effects on the mechanical properties of nano-crystalline materials can be evaluated clearly. This research first compares the results of crack growth behavior in single crystalline Fe for three typical interatomic potentials (Embedded Atom Method (EAM), Finnis Sinclair (FS), and Second Nearest Neighbor Modified EAM (2NNMEAM) potentials) and a Hybrid potential method, which uses FS potential for bcc structure atoms and 2NNMEAM potential for non-bcc structure atoms. The 2NNMEAM potential is accurate, but the computation time is dozens of times that of FS potential, which is the simplest of the three interatomic potentials. Therefore, the 2NNMEAM potential requires too much calculation for the purpose of this research that analyzes the crack growth behavior in nano-crystalline metals. However, Hybrid potential is able to give results similar to those of the 2NNMEAM potential, and the calculation time is close to that of the FS potential. From these results, the crack extension behavior in relatively large nano-crystalline models is analyzed using the Hybrid potential, and we demonstrate the grain-size dependency of the fracture behavior.