In the present paper we suggest the cause and solution of some unidentified X-ray diffraction (XRD) peaks inferroelectric nanoparticles. Indeed, a relationship between the structurally unfit XRD peaks and domains in the ferroelectricnanoparticles is suggested. BaTiO$_3$, PbTiO$_3$ and Sr$_{0.5}$Ba$_{0.5}$Nb$_2$O$_6$ nanoparticles were used as trial samples. Diffraction of X-rays by domain grating was considered for the occurrence of unfit peaks. It was found that domain widths corresponding to some structurally unfit minor peaks of all three trail samples show good agreement to the values estimated from the transmission electron microscopy images. The study can be used to estimate the width of nanodomains (within 5–10 $\AA$) in ferroelectric nanoparticles. Thus, the study seems to be highly important for the advancement of nanoferroelectricity.