Nanocrystals of 1-phenyl-3-((dimethylamino)styryl)-5-((dimethylamino)phenyl)-2-pyrazolin e (PYB) with different sizes were prepared by the reprecipitation method. The aggregate formation of PYB nanocrystals was found to be J-aggregate, of which the wavelength of absorption peak was observed at 445 nm. Further, we found that the PYB nanocrystals possessed a special optical size-dependent property. As the nanocrystal size decreased, the molecular absorption peak of PYB nanocrystals was observed to shift to high-energy side due to size effect and the J-aggregate absorption peak gradually vanished simultaneously.