Polyimide/diamond nanocomposites have been synthesized from 4,4'-diaminodiphenyl ether (ODA) and 3,3',4,4'-benzophenontetracarboxylic dianhydride (BTDA). All the polyimides show non-crystalline X-ray diffraction. The frequent occurrence of particular interatomic distances (R) denoted by the non-crystalline X-ray diffraction maxima are determined. An ultramicro-indentation technique is employed to evaluate the effects of nano-diamond particles on the indentation behavior of polyimides. Indentation size effect is observed and discussed. {graphics}