The number of metastable state densities obtained ranges from 1×1015to 4×1015 m-3 in the electron cyclotron resonance (ECR) argon plasma by measuring the optical absorption of the 8115 nm spectral line under the conditions of 02—08 Pa and 500—800 W. Taking into account the c ontribution of both the ground state and the 1s5 metastable state to the emiss ions, the electron temperature Te is measured by the ratio of emissio n lines. Results show that there are obvious differences compared with only considering t he metastable state densities.