The tensile behaviors of FCC Ni-Fe alloys were investigated within three grain size regimes: > 100 nm, 15-100 nm, and < 15 nm. The results show that the nanocrystalline metals demonstrated large strain hardening rates, which increase with decreasing the grain size. With the similar grain size, lowing the stacking-fault energy (SFE) by addition of alloying element increases the yield strength and strain hardening ability. The "low" tensile elongation of nanocrystalline metals is due to the basic tradeoff between the strength and tensile elongation, i.e. nanostructured metals are not inherently brittle. Both the tensile results and fracture surface observations suggest that the tensile ductility increases with increasing the grain size. Furthermore, within the large grain size regime, the fracture surface exhibited the real void structure; while the fracture surface showed the concave and convex features when the grain size is less than the critical value.