The equation of polymerization rate of L-alanine NCA initiated by n-butylamine in acetonitrile was determined by measuring the CO₂ evolved in the reaction. Though the reaction was heterogeneous as acetonitrile was nonsolvent for poly-L-alanine, the rate of polymerization could be represented as second order rate equation which was first order with respect to monomer and initiator concentrations, respectively. The polymerization rate of L-alanine NCA proceeded fast in early step from initiation until about 70 minutes but slow in the next step, and in each step, the reaction rate constants were obtained at temperature range from 0℃ to 35℃. The activation energies in fast and slow rate step were 3.7 kcal/mole and 4.1 kcal/mole, respectively.