The base-catalyzed rapid polymerizati on of caprolactam applied to the spinning of synthetic fiber was investigated．Acetanilide was used as molecular weight stabi-lizer，which could efficiently control the course of polymerization and to obtain the polymer with stable viscosity．The scale-up experiments on continuous polymeriza-tion．and direct spinning were run smoothly．The spun multifilaments had tensile strength vf 5—6 g/denier and elongation of 19—25％，comparable to the same grade nylon 6 product from the hydrolytic polFmerization．The time of polymerization wag two to two and half hours，while in the ordinary hydrolytic polymerization process，20一30 hours or more was necessary．