N-alpha,N-epsilon-(Phenoxycarbonyl)-L-lysine methyl ester 1a can be expected to behave as a difunctional blocked diisocyanate monomer derived from L-Iysine with diphenyl carbonate. In this report, we examined a reaction of the obtained compound 1a and 1 equiv of n-butylamine as a model reaction to find that the N-alpha position of compound 1a reacted selectively with an amine. This means that the reactivity of N-alpha- and N-epsilon-phenoxycarbonyl groups is different because the N-alpha-phenoxycarbonyl moiety has an adjacent ester group. From this result, synthesis of the corresponding polyureas was evaluated by a stepwise reaction of compound 1a with various diamines. The structure analysis and the study of thermal properties of the synthesized polyureas suggested that the carbodiimide moieties were partly constructed in the main chain because of the dehydration triggered by heat treatment of the polyureas.