Structural characterizations reveal that the plasma polymerization of three cyanate ester monomers proceeded mainly via the opening of p-bonds of the cyanate ester functional groups and transforming them to a large p-conjugated structure, which is noticeably different from the conventional thermal or catalytic cyclotrimerization reaction of cyanate ester monomers. The dielectric properties of the resulting plasma thin films were evaluated and results show that the dielectric constant of the three polycyanurate thin films decreased with increasing frequency, while in contrast, the dielectric loss factor increased with the increasing frequency.