Octyl urea has one solid-solid phase transition at the temperature of 353.2 K. The solid-solid phase transition of octyl urea has a large entropy, that is comparable to its melting entropy. In this study, octyl urea was examined by solid-state NMR in order to investigate the sources of the large transition entropy. The NMR results indicate that conformational exchange and molecular motion in the octyl urea are the sources of the large entropy of the solid-solid transition.