Periodic surface nanostructures formed on nitrided stainless steel with femtosecond (fs) laser pulses were investigated and compared with those of an untreated surface. The nanostructures are formed in the direction perpendicular to the laser polarization, and the variation of the mean spacing D in the nanostructures was examined as a function of the laser fluence F and the shot number N of fs laser pulses. The minimum value of D observed at F around the ablation threshold was 250 nm, which corresponds to approximately 1/3 of the laser wavelength lambda, and D increased with increasing F. The characteristic phenomenon where an irradiated area on the nitrided surface swells was observed. The size of D formed over a broad area for the nitrided surface was 10-20% smaller than that for the untreated surface. (C) 2012 Elsevier B. V. All rights reserved.