The parameters of an electron cyclotron resonance (ECR) C4F8/Ar plasma were measured with a heated Langmuir probe and an 8 mm microwave interferometer. As the ratio of C4F8 gas to Ar gas was increased the electron density measured with the microwave interferometer decreased, which suggests that a large quantity of negative ions exist in the ECR C4F8 plasma. An attempt to estimate the negative ion density was then made with the heated Langmuir probe. It was found that the ratio of negative ions to positive ions is from 30 to 80% in the ECR C4F8 plasma.