Polytypism and solid-state phase transitions of petroselinic acid (cis-6-octadecenoic acid) have been investigated with microscopic observation and micro-FT-IR spectroscopy. Polytypism was confirmed for the low-melting (LM) phase. In solution crystallization of the LM phase, polytypic transformation from a single-layered polytype to a double-layered one was observed. The LM phase performs a solid-state phase transition to the high-melting phase through two mechanisms. One mechanism involves the thermally induced nucleation and growth of the product phase, which is dominant upon a slow heating process. The other mechanism can be regarded as a martensitic transition, where rapid cooperative molecular displacements are initiated by a local stress. The martensitic transition occurs when a single crystal of the single-layered polytype suffers rapid heating or mechanical stress. However, no martensitic transition was observed for the double-layered polytype. The difference in transition behavior between the two polytypes was interpreted on the basis of crystal structures.