Low-dielectric-constant SiCOH films fabricated using plasma enhanced chemical vapor deposition (PECVD) are widely used as inter-metallic dielectric (IMD) layers in interconnects of semiconductor chips. In this work, SiCOH films were deposited with 1,1,1,3,5,7,7,7-octamethyl-3,5-bis(trimethylsiloxy)tetrasiloxane (OMBTSTS), and plasma treatment was performed by an inductively coupled plasma (ICP) system with mixture of He and H2. The values of relative dielectric constant (k) of the as-deposited SiCOH films ranged from 2.64 to 4.19. The He/H2 plasma treatment led to a reduction of the k values of the SiCOH films from 2.64-4.19 to 2.07-3.94. To investigate the impacts of the He/H2 plasma treatment on the SiCOH films, the chemical compositions and structures of the as-deposited and treated the SiCOH films were compared by Fourier transform infrared spectroscopy. The experimental results indicate that the k value of the SiCOH films was decreased, there was a proportional increase in pore-related Si-O-Si structure, which is commonly called the cage structure with lager angle than 144°, after He/H2 plasma treatment. The He/H2 plasma treatment was considered to have reduced the k value by forming pores that could be represented by the cage structure. On the other hand, the leakage current density of the SiCOH films was slightly degraded by He/H2 plasma treatment, however, this was tolerable for IMD application. Concludingly the He/H2 plasma treated SiCOH film has the lowest relative dielectric constant (k~2.08) when the most highly hydrocarbon removal and cage structure formation increased.