A simple method for spectral line broadening function (BF) determination of WUMa-type binaries is presented. The method uses the definition of the BF as the kernel in the convolution leading from a sharp-line spectrum to the rotationally broadened spectrum. It uses the overdetermined set of linear equations and requires handling of large arrays. The solution incorporates an orthogonalization step in order to remove linear dependencies introduced by the presence of the featureless continuum in the sharp-line spectrum. The method is simple in concept and can be used for any problems requiring determination BFs which are narrower than the used spectral window. The application of the method is illustrated for AW UMa.