The phase transition of NaNO2, is investigated by using X‐ray powder diffractometry. The temperature dependence of the lattice constants, peak intensities, and the order parameter of the ferroelectric phase, in the range of 27 to 200 °C, show the same results as obtained by using single‐crystal diffractometry. These experimental findings imply that the X‐ray powder method is suitable for the study of phase transition. The analysis of the powder diffraction profile would even improve the method.