The polymerization of $varepsilon$-caprolactam has been carried out with the three additives; water, acetic acid and butylamine. Rate of polymerization was accelerated by the additives and the viscosity of polymer initiated by acetic acid or butylamine at the equilibrium state was lower than that by water only. The number of end groups was first increased with polymerization time and the slightly decreased. The number of amine end groups at the equilibrium state was decreased as the following order; butylamine, water and acetic acid, but the number of carboxyl end groups was increased in the reverse order. In the dyeing test with the dyestuffs of acid milling type and levelling type, initial dyeing rate of filaments was proportional to the number of amine end groups and also increased in the following order of additives; butylamine, water and acetic acid.