In the past the powder diffraction data where presented as d-I sets as obtained experimentally and systematic errors were utilized only for the derivation of the unit cell parameters. This attitude was justified by the fact that the major work of XRPD made by Debye-Scherrer camera and it was assumed that most users would obtain the same systematic errors. Nowadays, diffractometry has taken over, and the diffractometers have lower systematic errors, which can minimized by calibration. Thus, they are now preferred. There are many phases which can be used as standards, but only four were selected, namely, Si, Ag, W, and mica (FP), which can easily be obtained as pure substances, have a limited number of diffraction lines and the distribution of intensities along 2θ is good. The calibration is made by fitting a polynomial which correlates the standard experimental peak positions versus the expected (calculated) values. However, while on the one hand, the more peaks which are used, the better the fit which can be achieved; on the other hand using a standard with many peaks enhances the probability for interference with the examined specimen peaks. Thus, it was decided to determine each line position by line profile fitting as was recommended elsewhere. In order to derive real observed data we do not link the diffraction lines between themselves by global structural or tine shape parameters. Thus, local variations in line profile parameters are treated. In the line-profile-fitting method suggested here the final structural details (atomic positions) are not required. Similar method have been used in the past for structure analysis, unit-cell refinement, and broadening analysis of pure substances. It was found that using the suggested method yields accurate unit cell parameters for each individual phase in the polyphase mixture. Several systems will be demonstrated in this work.