Abstract: To investigate the technique of laser etching silicon under different mediums,micromachining of silicon was conducted using a short-pulse(FWHM is 20ns) KrF excimer laserwith energy of 150mJ~250mJ.Laser etching were tested on a silicon workpiece both in air and under water.Based on the results of experiments,basic etching appearance and etching velocities by excimer laser etching silicon under the two mediums were studied,and then the differences were compared.As a result,the molten material is easy to be removed in water-assisted micromachining of silicon,which helps to improve the processed surface quality;and the restriction of water improves the impact effect which increases the etching velocity.