Laser-induced periodic surface structures by femtosecond laser pulses become a good technology for producing a large variety of structured ma-terials while the formation mechanism of fs laser-induced periodic tipples is still an open problem.The formation dynamics of periodic ripples induced byfemtosecond laser pulses are studied by a collinear pump-probe imaging technique,in which a 100× microscope is used to promote the spatial resotutionto 440 nm.