Anisotropic reactive ion etching of silicon has been widely used in fabrication of diffractive optical elements, waveguides and structures of Micro-Electro-Mechanical Systems (MEMS) in recent years. It is important to achieve minimal roughness on both horizontal surfaces and sidewalls in reactive ion etching (RIE) of silicon. Now reactive ion etching of silicon in an RF parallel plate system, using SF6/O2/CHF3 plasmas, has been experimentally studied. Black silicon method (BSM) is a powerful tool that can be used to find the best conditions under which the anisotropic etching of silicon is optimized. In the process of etching silicon, SF6 serves as the major reactive etching species on silicon, while O2 is added to build the passivation layer on the sidewalls and surfaces to get anisotropic profiles and CHF3 to suppress the formation of passivation on horizontal surfaces in order to achieve smooth etch surfaces. BSM provides a convenient way to know the function of the gases adding into the RIE and to find how to adjust the proportion of the gas combination. Experimental parameters including the etching rate, selectivity, anisotropy, and self-bias voltage under different parameters have been examined through SEM and AFM. As a result, anisotropic etching conditions and the parameters corresponding to the minimal roughness on the surfaces were obtained.