The equation of polymerization rate of glycine NCA in acetonitrile, initiated by n-butylamine, was expressed as the first order with respect to monomer and initiator concentrations, respectively. Two stage rate profile was observed : the fast first stage (50 minutes after initiation) and followed by slow second stage. The reaction rate constants were determined for fast and slow stage reactions. The activation energies in fast and slow rate step were 3 kcal/mole and 4 kcal/mole, respectively. The mechanism of chain growth was also discussed.