Laser-induced periodic nanostructures have been observed in several kinds of material including dielectric, semiconductor, metal and polymer. Fabrication of nanostructures on 6H SiC crystal illuminated by 800 nm and 400 nm femtosecond laser is studied. Interference ripples with period of 150 nm and 80 nm are produced by linearly polarized 800 nm and 400 nm femtosecond pulses, respectively. Spherical nanoparticles with diameter about 100 nm are formed after irradiation with circularly polarized 800 nm femtosecond pulses. Alignment of nanoparticles about 100 nm is obtained after simultaneous irradiation with 400 nm and 800 nm linearly polarized femtosecond laser pulses. The direction of the alignment changes to the polarization direction of 400 nm incident laser pulses, along with the increase of its pulses energy. With the help of the second harmonic generation, the formation mechanisms of these nanostructures are discussed.