Plant starches have A-, B-, and C-type crystallinity. We investigated spectrum characteristics of starches from rice, potato, pea and lotus by X-ray powder diffraction (XRD). Peak-fitting, curved-line, straight-line mapping and curved-line mapping methods are commonly used for determining relative crystallinity of starch XRD spectra. The curved-line mapping method has reliable results. We found the relative crystallinity of rice starch negatively associated with amylose content and that of acid-modified lotus starches positively associated with acid hydrolysis degree by the curved-line mapping method. The crystallinity of lotus starch changed from C- to A-type during acid hydrolysis. These results would be useful for XRD analysis of crystallinity and relative crystallinity of plant starches.