Abstract: Experiment on cylindrical liquid jet containing de-ionized water and SiO, nanoparticles impacting obliquely on a single crystal silicon surface at a speed of 50 m/s was perforrned. Microstructure of the impacted silicon surface was examined through a high resolution transmission electron microscopy, an atomic force microscopy, etc. Some crystaI defects, lattice distortion, and grain refinement and rotation of grains in the surface layer after exposing for 30 seconds have been observed. At exposure time of 10 minutes, an amorphous layer containing a few of crystal grains exhibits in the subsurface, and lots of craters, scratches, and atom pileups can be found in the surface. Surface damage aggravated with increase of exposure time.