Formation of carbamates by amino groups of poly(ε-l-lysine) (ε-PL) and cross-linking of ε-PL were studied by using 13C and 15N solid-state NMR. It is a characteristic found in ε-PL cast from basic aqueous solution exposed to the air or gaseous CO2. It is not observed in ε-PL cast from acidic aqueous solution and ε-PL cast from degassed aqueous solution under CO2 free environment. The carboxyl carbon and amide nitrogen appear at 164 ppm in 13C spectrum and 92 ppm in 15N spectrum, respectively, which arise when some amino groups of ε-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 ε-PL.