We observe the morphological change and grain structure of Ni foil when it is ablated with femtosecond laser pulses. Scanning electron microscopy and field emission transmission electron microscopy are used to study the nature of the morphology and grain structure of nickel foil and determine the essential features. The results indicate that there are many random nanostructures in the center of the ablated region composed of nanocrystalline grains as well as some core-shell structures. The observed morphologies seem to suggest that phase explosion and extremely high cooling rate are the most probable physical mechanisms responsible for the formation of surface nanostructures.