In this study, atomic layer etching of Si has been carried out using $Cl_2$ adsorption followed by the irradiation Ar neutral beam of low energy. In this experiment, the etch rate of Si was dependent on the $Cl_2$ pressure(the surface coverage of chlorine) and the irradiation time of Ar neutral beam(the flux density of Ar neural beam). And the etch rate of Si(100) and Si(111) were saturated exactly at one monolayer per cycle with $1.36{AA}/cycle;and;1.57{AA}/cycle$, respectively.