This paper introduces a new method to determine the crystalline fraction in samples containing amorphous phases from experimental X-ray diffraction data. Computer generated codes, one for each measured data point, are used to interpret the pattern as to where diffraction peaks exist and what is the angular breadth of each peak's intensity above background. Two parameters are defined that are used to identify the position and intensity of the crystalline phase diffraction peaks. For mold fluxes used in continuous casting, the crystalline fraction of solid slag film is a key factor that can affect heat transfer between solidified shell and mold. In this work, a new method was developed to determine the crystallinity of solid slag films. This method does not require structure parameters or other references, and results can be obtained directly by reading a text file with diffraction data. Results indicate that, there is a positive correlation between crystalline fraction and integrated intensities corresponding to crystalline phases. The selection of integration interval does not have much effect on results. To simplify computations, 20–45°2θ was considered as an appropriate interval.