In a capacitively coupled discharge with external electrodes, hexamethylcyclotrisiloxane (D,3,) was polymerized in the presence of ammonia or nitrogen. The polymer structure was investigated by IR spectroscopy and XPS, and properties of the polymer were studied by TG analysis and contact angle detection. The results show that both N,2, and NH,3, participated into reaction and eventually incorporated into polymer structure mainly in the form of C=NH, and the N/Si ratio increased with increasing NH,3, concentration in the D,3,/NH,3, mixture. The polymeric films prepared in NH,3, are possessed of good hydrophobicity.