The paper presents the investigations of an atmospheric-pressure argon plasma generated at 915 MHz microwaves using the optical emission spectroscopy (OES). The 915 MHz microwave plasma was inducted and sustained in a waveguide-supplied coaxial-linebased nozzleless microwave plasma source. The aim of presented investigations was to estimate parameters of the generated plasma, that is, excitation temperature of electrons T exc , temperature of plasma gas T g , and concentration of electrons n e . Assuming that excited levels of argon atoms are in local thermodynamic equilibrium, Boltzmann method allowed in determining the T exc temperature in the range of 8100-11000 K. The temperature of plasma gas T g was estimated by comparing the simulated spectra of the OH radical to the measured one in LIFBASE program. The obtained T g temperature ranged in 1200-2800 K. Using a method based on Stark broadening of the H β line, the concentration of electrons n e was determined in the range from 1.4 × 10 15 to 1.7 × 10 15 cm −3, depending on the power absorbed by the microwave plasma.