A single-line analysis of X-ray diffraction patterns was improved to study the microstructure of boron doped diamond films. This analysis requires that an analytic function be ascribed to each reflection. Such a function must model the observed data as precisely as possible and should allow readily a separation of the breadth of convoluted functions. The line profile due to size effect is assumed to be a Cauchy function and the shape of the strain profile is taken as a Gauss function.