Solid-state phase transitions of pentadecanoic acid have been investigated with microscopic observation and micro-FTIR spectroscopy. Two martensitic phase transitions have been found; one is a transition from the B' form to the A' form, and the other is a transition from A' to B'. These transitions are initiated by a mechanical stress. The product phases grow in a definite direction and are highly oriented. The occurrence of these transitions depends on temperature. Below 30 degrees C only the transition from B' to A' takes place, while above 30 degrees C the direction of the transition is inverted.