Antireflective subwavelength grating was designed by using rigorous coupled wave approach(RCWA). The square-pillar grating was fabricated by plasma assisted etching. Testing results show that the grating has a very good antireflective characteristic, and the values of testing parameters approximately equal to the designed data. It indicates that the plasma assisted etching method is valid to fabricate deep groove grating. The experimental results are analyzed and discusssed. It is shown that the critical periodic point as a function of refractive index is very important to fabricate the antireflective aubwavelength grating.