The influence of diffraction of subwavelength relief grating on antireflection and transmission performance is investigated. Theoretical analysis shows that, for achieving both effects of antireflection and transmission enhancement, the period of grating must be small enough to eliminate the lateral waveguide loss caused by diffraction. In order to achieve antireflection effect only, the lateral waveguide caused by grating diffraction can be full used to reduce the reflectivity. Small grating period is not required at this point. The effects of period, duty cycle and groove depth of one dimensional relief gratings on reflectivity are investigated with the method of rigorous coupled-wave theory. A one-dimensional relief grating with period of 290 nm is fabricated by two-beam interference method. Experimental results show that it has significant antireflection effect in the whole visible light wave band and wide angular range, but this effect is only reflected in the non-diffraction region. The fabrication difficulty can be reduced greatly by choosing a reasonable grating periods according to the applications, which plays a positive role in using subwavelength gratings more economically.