In order to investigate the crystallization in silk fibroin induced by acid treatment and its photo-yellowing reduction effect. silk fibroin were immersed in PH 3.5~4.0 sulfuric acid and 0.01N hydro-chloric acid at 2$0^{circ}C$, and the changes of crystallinity were measured by infrared spectroscopy, X-ray diffraction. Yellowness index caused by UV irradiation were also observed. The results are as follows : 1. ${eta}$-form transition occurs when the silk fibroin are immersed at 2$0^{circ}C$ in pH 3.5~4.0 sulfuric acid and 0.01N hydro-chloric acid. Formation of new hydrogenbond in molecules on the amorphous region induced ${eta}$-form structure, and increased crystallinity. 2. Crystallization owing to $eta$-form transition reduced the initial yellowness index by ultraviolet irradiation.