The conversion curve of solid-state polymerization in polydiacetylene (PDA) nanofibers (NFs) was so different from that of PDA nanocrystals (NCs). PDA NFs clearly showed the longer induction period, and the conversion rate was much higher than that of PDA NCs. These differences have been discussed on the basis of thermally softened crystal lattice in PDA NFs and/or PDA NCs. In addition, the excitonic absorption peak position of PDA NFs was red-shifted, compared with PDA bulk crystals. This fact suggests that PDA chains inside PDA NFs would be almost strain-free and the effective pi-conjugation is fully extended possibly to the maximum level.