This paper investigates the effect of Si—OH groups on dielectric property and leakage current of the SiCOH low dielectric constant films deposited by decamethylcyclopentasioxane (D5) electron cyclotron resonance plasma. The results show that the increasing of Si—OH content in the films can lead to the increasing of dielectric constant k, decreasing of leakage current and rise in dielectric dispersion. The increasing of k value is the result of compensation of the decreasing of k value originated from cage by the strong polarization of Si—OH groups. The decreasing of leakage current at high Si—OH content is due to the low connecting probability p of networks because the networks break at the terminal Si—OH groups. In the case of high ionization degree of D5 plasma, more Si—OH groups break and form Si—O—Si linkages by chemical condensation occurring between proximal Si—OH groups. As a result, the k value of SiCOH films can be further reduced.