ATMS was selected as a precursor for SiCOH composite films and it is expected that allyl group and three methyl groups attached on silicon could be unitized effectively on a molecular scale to make SiCOH films along with CxHy imbedded in it. Direct plasma reactor was used where radical oxygen was generated in the chamber. The refractive index and film thickness were measured and calibrated with an ellipsometer and a FE-SEM. Chemical bonding and compositions of films were investigated using FT-IR and XPS, respectively. The other properties of films have been investigated by SIMS, TEM, AFM and nanoindentor, LCR meter, Hg probe. SiCOH composite films have been prepared using ATMS as the skeleton precursor and a porogen precursor. The porogen has been removed from the deposited films by thermal annealing at 420°C, obtaining films with dielectric constants down to 2.2. It was found that the properties of the annealed films depend on the deposition temperature and concentration of porogen.