Modulation grating has been achieved by two interfered femtosecond laser pulses on the surface of the silica glass. The modulation grating formed at the middle of each bulge of the common grating and was attributed to the higher-order modulation arising from second-harmonic generation (SHG) of the femtosecond laser pulse incident to the surface of silica glass. The periods and depths of the fundamental grating and the modulation grating have been observed by using an atomic force microscopy (AFM). Experimental results show that the average depth of the modulation grating is nearly a half of the depth of the fundamental grating (c) 2007 Elsevier B.V. All rights reserved.