A buried waveguide in LiNbO3 crystal is fabricated,by using the femtosecond laser which repetition rate is 25 MHz.The etching region can be divided into two regions which are inner interaction region and thermally modified region respectively.The coupling experiment shows two single mode buried waveguides are generated in the thermally modified region,and the propagation loss of the two waveguides are below 1.5 dB/cm.At the same time,the inner interaction region leading to a low transmittance.The Raman spectrum of the etching region is measured which shows the light-guide region is densificated by the thermal effect of high repetition,and there is a large-scale defects in the inner interaction region.