The commercial hydrogen silsesquioxane (HSQ) is made up from low-density and low dielectric constant (low-k) materials. In this paper, low-k HSQ films are obtained by spin-on deposition (SOD) and then followed by treatment with CH₄ plasma using electron cyclotron resonance (ECR). Fourier-transform infrared spectroscopy (FTIR) is used to identify the network structure and cage-like structure of the Si-O-Si bonds and other bonds. Capacitance-voltage (C-V) measurements are used to determine the dielectric constants. A slow positron beam is conducted in order to investigate the pore connectivity within these films. The results suggest that the dielectric constant of the HSQ films, which were treated with CH₄ plasma, increased a little after annealing. CH₄ plasma treatment can restrain the increase in the dielectric constant of the HSQ films.