The parameters of an electron-cyclotron resonance (ECR) C 4 F 8 /Ar gas plasma were measured using both a heated Langmuir probe and an 8-mm microwave interferometer. An attempt to estimate the negative ion density was made with three methods in the case where C 4 F 8 and Ar gas mixture rate was less than 25%; the first method is based on the analysis of the probe characteristics, and the others on the charge neutrality condition. It was found that the ratio of the negative ion density to the positive ion density was 20-50%, depending on the assumed positive ion mass. Furthermore, the self-bias effect in C 4 F 8 /Ar ECR plasma was discussed.