Our theoretical calculation has shown an empirical exponent law of the static second-order hyperpolarizabilities gamma in armchair and zig-zag nanotubes, i.e., gamma/gamma(60)=(U/60)(P), where U is the number of carbon atoms in nanotube, gamma(60) is the static gamma value of C-60, and P=3.15 and 2.98 for armchair and zig-zag nanotubes, respectively.