Formation of carbamates by amino groups of poly(epsilon-l-lysine) (epsilon-PL) and cross-linking of epsilon-PL were studied by using C-13 and N-15 solid-state NMR. It is a characteristic found in epsilon-PL cast from basic aqueous solution exposed to the air or gaseous CO2. It is not observed in epsilon-PL cast from acidic aqueous solution and epsilon-PL cast from degassed aqueous solution under CO2 free environment. The carboxyl carbon and amide nitrogen appear at 164 ppm in C-13 spectrum and 92 ppm in N-15 spectrum, respectively, which arise when some amino groups of epsilon-PL react with gaseous CO2 to make carbamates. In addition to these peaks a peak at 171 ppm appears. We assigned it to amide C=O carbons which can not make intermolecular hydrogen bondings since there exist bulky carbamates groups close to these C=O groups. Self-assembly of ionic pairs of ammonium groups and carbamate anions leads to cross-linking of epsilon-PL.