Organic nanoparticles of 2-pyrazoline were prepared by the method Their excitonic transitions responsible for absorption and, emission as compared with those of dilute solution, have been investigated as a function of nanoparticle size. We found that pyrazoline nanoparticles possess a special size dependence in their optical properties. As the nanoparticle size decreased, Me absorption, and emission. peak of pyrazoline nanoparticles were observed to shift to the high-energy side due to size effect.