Polyp-oxybenzoyl) (POB) crystals are prepared by the polymerization of p-acetoxybenzoic acid (pABA) at 320 degreesC in various solvents to clarify the influence of miscibility between oligomer and solvent on the morphology as well as the size of the crystal. Concerning the morphology, whiskers are formed in less miscible solvents such as liquid paraffin and Barrel process oil B27. On the other hand, bundle-like aggregates of fibrillar crystals tend to be formed in the solvents having higher miscibility such as Therm S 900 and 800, which are aromatic heat exchange media. Further, the solvents having higher temperature at which p-ABA is completely dissolved during polymerization (TS) yield a larger number of the whiskers with smaller width. The higher supersaturation of oligomers in less miscible solvents causes the formation of a larger number of nuclei with smaller size. Bundle-like aggregates are formed in the solvents whose T-s is lower than 140 degreesC by the fibrillation of pillar-like crystals during polymerization owing to the reorganization. This shows a clear relationship between the width and T-s, and between the number of the crystals and T-s. Hence, the size of the whisker such as the length and the width can be predicted by the calculation with Ts, which is very valuable for the preparation of POB whiskers for use as industrial materials.