We investigated the etch characteristics of Si in Cl-2 plasmas with addition of fluorine in an electron cyclotron resonance etching system. The addition of fluorine-containing gas, SF6, increased the etch rate and selectivity, whereas anistropy was decreased. The ion-assisted etch mechanism was dominant in a pure Cl-2-gas plasma, whereas chemical etching was dominant at the ratio of 0.65 of Cl-2/total gas flow rate (Cl-2 + SF6). In the transition region, 0.65-0.9, the small addition of the fluorine-containing gas brought about the generation of fluorine radicals, and this can chemically detach the chlorinated layer on feature sidewalls. Anistropy was abruptly reduced by this chemical etching.