B-C-N thin films of a wide composition range were deposited by reactive d.c. magnetron sputtering from targets with different B/C ratio in an Ar/N-2 atmosphere. The films were investigated by electron diffraction (ED) and their mechanical properties hardness and Young's modulus were determined from nanoindentation. The diffraction pattern of most of the films gave evidence of an amorphous structure and the number and distance of nearest neighbours were calculated. The mechanical properties hardness and Young's modulus are correlated to the structural parameters. An empirical formula could be established that allows the calculation of hardness and Young's modulus when the composition of the film and the number and distance of nearest neighbours are known. In this formula, the percentage of each element (B,C and N) is included in a weighted sum that is close to the average valence electron number. (C) 2004 Elsevier B.V. All rights reserved.