Influence of acyl end-group of oligomers on morphology of poly (p-mercaptobenzoyl) (PMB) was examined by the polymerization of S-acyl-4-mercaptobenzoic acids having the different acyl groups and their dimers in liquid paraffin at 260-300 degrees C. The polymerization of S-acetyl-4-mercaptobenzoic acid afforded PMB whiskers, whereas the polymerization of S-propionyl-4-mercaptobenzoic acid, S-butyryl-4-mercaptobenzoic acid and S-valeryl-4-mercaptobenzoic acid yielded PMB microspheres. The microspheres were formed by the liquid-liquid phase separation of oligomers induced by the longer acyl end-groups than acetyl group of the oligomers due to the reduction of freezing point of oligomers. In contrast to this, the polymerizations of their dimers afforded needle-like and slab-like crystals instead of microspheres. In these polymerizations, the crystallization was induced due to the increase of the molecular weight of the phase-separated oligomers, leading to the increase of the freezing point of oligomers. The morphology of PMB could be controlled by the chemical modification of oligomer end-groups.