Microcrystals of some diacetylenes, prepared by the reprecipitation method, have been studied as dispersions in liquid media. Interesting behavior has been observed in the solid-state polymerization of diacetylene monomers and with the optical properties of polydiacetylene (PDA) microcrystals. First, the polymerization perfectly proceeded from one end to the other end of the diacetylene microcrystals. Next, the excitonic absorption peak position was found to shift to higher energy side with decreasing size of the PDA microcrystals. The size effect was observed even for crystals as large as 100 nm or more in contrast to conventional quantum effect of inorganic semiconductors where size effect is observed only for microcrystals of less than about 10 nm size. In addition, since the microcrystal dispersions in water have low optical loss, the optical Kerr shutter response of PDA microcrystals could be measured, and the non-resonant X(3) value was estimated to be on the order of 10-13 to 10-12 esu in very low concentrations (ca. 10-3 M).