Molecular Stark effect in NH2D is used to modulate strong P (20) line at 10.6 pm of a CO2 laser. A modulation depth of 41% is obtained from a 400 Hz 20V rms sinusoidal modulation signal and a 440 V DC bias applied to a 20 cm gas cell outside the laser at a gas pressure of 5 torr. The maximum modulation depth of the gas cell is calculated as 42%, which is in good agreement with the experimental result. When sinusoieal modulation signal is 2 MHz 20 V rms, a modulation depth of 12% is achieved. NH2D was prepared by mixing ND3 and and ND3 was prepared by the present authors.