The purpose of this study was to investigate dyeing properties of silk and cotton fabrics by Alnus firma fruit extract. The properties were evaluated by its extracting condition, dyeing condition, mordants variables, methods of mordanting, and color fastness. IR spectrum showed absorption band O-H at 3400 $cm^{-1}$, ester >CO at 1730 $cm^{-1}$, C=O at 1600 $cm^{-1}$, aromatic C-C at around 1500 $cm^{-1}$, and ester CO=O at 1045 $cm^{-1}$. And the spectrum of Alnus firma fruit coincided with tannic acid spectrum, so Alnus firma might be hydrolyzable tannin. The wavelength of maximum absorption of extract was appeared at 270 nm and absorbances of extract were increased by extracting temperature and time condition. Silk fabric showed higher K/S value than cotton fabric at the same condition. Surface color of dyed fabrics was various according to the used mordants: AI, Sn mordanted fabrics were dyed in yellowish, Cr, Cu mordanted fabrics were brown and Fe mordanted fabric was purple blue. Water fastness of Fe mordanted fabrics was improved as compared with unmordanted fabrics.