Polymerization of N-carboxy anhydrides (NCAs) of L-valine and DL-valine in the solid state in hexane was compared with their polymerization in the anisole solution, and the results were explained with reference to their crystal structures. Both of L-valine NCA and DL-valine NCA were less reactive in the solid state than in the solution, but the molecular weight of the polymer of L-valine NCA obtained in the solid state was much higher than that in the. solution. The molecular weight of the polymer obtained in the solid state polymerization of DL-Valine NCA increased with the increase in conversion, while that in the solution decreased with the conversion. Electron microscopy, X-ray analysis and IR spectroscopy suggested that : the polymerization of L-valine NCA and DL-valine NCA took place in the layer normal to the b axis in the crystal, which was suggested by their crystal structures. In addition, it was found that L-phenylalanine NCA and DL-phenylalanine NCA were very reactive in the solid state, while they were inactive in the solution.