In this study we develop a method which utilizes the linewidth of the diamond peak at 1332 cm−1 to determine the diamond domain sizes. We have carried out calculations based on the phonon dispersion curves of diamond and the breakdown of wavevector selection rules in Raman scattering. The linewidths of a series of diamond samples were calculated, and from this theory the dimensions of the diamond regions were obtained. The calculated domain sizes ranged from 57 to 100Å and were compared to values obtained from SEM micrographs which were between 5000 and 25000Å. This is consistent with other results which have found that domain sizes calculated from this theory are in general consistent with sizes determined from electrical conductivity measurements, not from microscopy techniques. In addition, this theory allows for the calculation of the amount of frequency shift due to microcrystallinity, so that the residual stress in the film can be found.