Nanodiamonds produced by detonation and purified by oxidation have a large number of different oxygen-containing surface groups. A full control of surface chemistry is critically important in all applications of nanodiamonds. In particular, it has been found that nanodiamonds can be used as radical trapping stabilizers in thermal processing of polymers. To explain their radical trap action, we hypothesize that nanodiamonds have functional groups which are similar to conventional antioxidants, hindered phenols, but are stable up to 350 °C. In this research, the thermal stability of nanodiamond-polyether ether ketone (PEEK) composites was investigated. PEEK is well-known as one of the best engineering plastics with excellent mechanical and chemical resistance properties. However, at high temperatures, unwanted radical induced reactions such as cross-linking or extension of the chains occur, resulting in deteriorations of processibility and performances of the polymer. To date, no suitable thermal stabilizer has been found to suppress these undesired radical reactions. This is because conventional antioxidants are unstable over 350 °C, which is the processing temperature of PEEK. We developed a novel high temperature stable nanodiamond-PEEK composites. Rheometry and thermal gravimetric analysis (TGA) were used for characterization of the composites. Furthermore, Boehm titration was carried out to determine the phenolic hydroxyl groups on the nanodiamond surface, which are important functional groups to support our hypothesis about the mechanisms of nanodiamond induced thermal stabilization of polymer composites.