This paper investigates the effect of vacuum thermal treatment on current-voltage (I-V) and capacitance-voltage (C-V) characteristics, hydrophobic properties and microstructure of CH4 doped SiCOH low dielectric constant films deposited by decamethylcyclopentasiloxane (D5) electron cyclotron resonance plasma. The results show that the desorption of thermally unstable CHx groups during the heat treatment can lead to the decrease of leakage current, the variation of SiCOH/Si interface state and the decrease of surface roughness. However, the desorption of CHx groups also leads to the deterioration of hydrophobic property.