By utilizing an external Doppler dither modulation scheme, the frequency of the CO2 laser 10P(16) and 10P(18) lines are locked at the centers of the saturated absorption lines of SF6 molecules in an external absorption cell. The locked laser has a frequency fluctuations of some 7 kHz. The looked laser output has neither parasitic frequency modulation nor parasitic amplitude modulation. The stabilized frquency is independent of laser operating characteristics.