PURPOSE: To obtain the subject hexagonal system crystal of diacetal providing a molding free from fish eye as a nucleating agent for an olefin resin, compris ing a diacetal such as 1,3:2,4-bis(O-3,4-dimethylbenzylidene)-D-sorbitol.

CONSTITUTION: 1,3:2,4-Bis(O-p-dimethylbenzylidene)-D-sorbitol of cubic system is obtained from D-sorbitol, etc., and p-methylbenzaldehyde, etc., using cyclohexane as a hydrophobic organic solvent, methanol as a polar solvent and sulfuric acid as an acid catalyst by a method for removing condensed water out of the system. Then the compound is dispersed into xylene, dissolved under heating at 140°C, filtered and the filtrate is allowed to cool and gelatinized. Xylene is evaporated from the gel at room temperature under vacuum to give the objective hexagonal system crystal of the formula (R1 and R2 are each a 1-4C alkyl or a halogen; (m) and (n) are each O-2; (p) is O or 1) providing a molding having excellent external appearance, optical, mechanical and thermal characteristics as a nucleating agent for a polyolefin-based resin.