Development of high-k composites with low dielectric loss and good flame retardancy is still a big challenge. Herein, unique cyanate ester (CE) composites based on multifunctional carbon nanotubes (EPHSi-g-MWCNTs) coated with phosphaphenanthrene terminated hyperbranched polysiloxane (EPHSi) were prepared. The 2.5EPHSi-g-MWCNT/CE composite with 2.5 wt % EPHSi-g-MWCNTs has the highest dielectric constant. The value at 100 Hz is about 1.4 times that of the 0.7MWCNT/CE composite that has the biggest value among MWCNT/CE composites, while interestingly, the dielectric loss at 100 Hz of the 2.5EPHSi-g-MWCNT/CE composite is only 5.8 X 10(-5) times that of 0.7MWCNT/CE. Also, the 2.5EPHSi-g-MWCNT/CE composite shows outstanding flame retardancy, reflected by the longer time to ignition, much lower peak heat release rate, and total smoke production. The study on the origin behind this demonstrates that, different from a simple combination of EPHSi and MWCNTs, EPHSi-g-MWCNTs have a super synergistic feature in preparing high-k polymeric composites with low dielectric loss and outstanding flame retardancy.