We give evidence of nanometric size (5-15 nm) crystalline diamonds in carbon thin films obtained at low substrate temperature(15 degreesC) under the action of low pressure (0.1-0.3 Torr) 35 kHz excited CH4/Ar (80-95%) plasmas. The decrease in Ar concentration was found to lead to higher film hardness while crystalline nanodiamonds are observed in a wide interval (1-3.5 h) of deposition times but only for very high Ar concentration (95%). The polycrystalline nanodiamond grains are found over 10-20% within an amorphous carbon matrix. It is suggested that the distribution of nanodiamond grains might be connected to the nonuniform ion energy distribution in the Ar-rich plasma generated at 35 kHz. Morphological and structural features of the deposited films were also investigated. (C) 2002 Elsevier Science B.V. All rights reserved.