Characteristic of the orthorhombic and tetragonal phase of (K, Na)NbO3-based lead-free piezoelectric ceramics was analyzed in theory. X-ray diffraction differences between the orthorhombic and tetragonal phase were clarified in the 2θ range from 20° to 60°. For the orthorhombic (K, Na)NbO3-based piezoelectric ceramics, the number and relative intensity of X-ray diffraction peaks were calculated from a monoclinic primitive cell. A method distinguishing orthorhombic and tetragonal phase from the X-ray diffraction patterns was proposed. It is found that the phases of (K, Na)NbO3-based piezoelectric ceramics can be distingwished exactly from peaks of the primitive cell planes (1 0 2) and (1 2 1). For the powder diffraction, the phases can be distinguished more easily by relative intensities of the diffraction peaks for the peak-split position at around 22° or 45°.