Periodic nano-grating structures self-formed on the surface of several metals by femtosecond laser pulses are analyzed by two-dimensional Fourier-transform. For the self-formation of grating structures on metal surfaces, the period of the structures depend on laser fluence. This dependence is the same for all metals, although the range of laser fluence in which the structures are formed differs between metals. For Ti with relatively low melting temperature(T-m=1938K) the grating nanostructures were self-formed for laser fluence of 0.09 - 0.45 J/cm(2), while for Mo with high melting temperature (T-m = 2888K) structures were formed for laser fluence of 0.18 - 1.1 J/cm(2). Higher the melting temperature, upper limit of grating structure formation tends to be higher. These experimental results indicate that the period of the self-formed grating structure depend not on metal characteristics, but rather on the density of the surface plasma produced by a laser pulse.