A filtered Thomson scattering technique has been developed to measure electron temperature T e and electron number density n e in an atmospheric pressure argon plasma. The problems of the strong background scattered light and the high plasma luminosity were resolved by employing an atomic vapor filter that provided strong attenuation of stray scattered light at the laser wavelength while simultaneously allowing the passage of the spectrally broad Thomson scattered light. A relatively low-power Ti: sapphire laser (about 50 mJ/pulse) was used in conjunction with an optically thick rubidium notch filer. High spectral purity of the laser was achieved by developing an amplified stimulated emission filter. Because of the geometrical constraints of the plasma arc source, experiments were performed in the backscatter geometry. Information on the electron number density and the electron temperature was derived from the Thomson signal by fitting a model curve to the data. The measurement results gave an electron temperature of 0.82 ± 0.06 eV and electron number density of 1.61 × 10 16 ± 0.05 X 10 16 electrons/cm 3