The effect of sericin on the crystallization and gelation of fibroin was studied. We used urea-mercaptoethanol for extraction of sericin from silkworm cocoons. The degradation of sericin during extraction could be minimized and the molecular weight of sericin was in the range of 10-400 kDa. Based on the result of time-resolved FTIR-ATR spectra, the crystallization of fibroin was retarded in the presence of urea-mercaptoethanol extracted sericin like as previously reported hot-water extracted sericin. The gelation time of fibroin was also delayed in the presence of sericin. Interestingly same delaying effect could be also observed when fibroin and sericin were immiscible. These results show that sericin might have more active role in the spinning process of silk fiber.