The A＇ form of pentadecanoic acid can be divided into two phases : A＇(h) and A＇(l). Under certain conditions, a single crystal of the A＇(l) phase performs a rapid reversible solid-state transition to the A＇(h) phase on heating, accompanying its morphological change in the whole region. During this transition single-crystal state is held completely. The transition from A＇(h) to A＇(l) and that of the opposite direction can be induced also by a mechanical stress. These transitions can be recognized as martensitic transitions. By a mechanical stress, the B＇ phase transforms to A＇(l) with two stages : B＇-->A＇(h) and A＇(h)-->A＇(l) transitions.