It is well known that the Petasites japonicus has been used for a long time medicine for the treatment of allergic diseases such as lacquer poisoning. However, the exact components and dyeing properties of its effects is still not known. Therefore, the purpose of this study was to investigate the dyeing property and ultraviolet-cut ability of silk and nylon fabrics that was dyed variously with the Petasites japonicus. The Petasites japonicus extract was done by boiling with distilled water at $100^{circ}C$ for 1 hour. As mordanting agent, we used Aluminum potassium sulfate ($AlK(SO_4)_2{cdot}12H_2O$), Copper(II) sulfate pentahydrate ($CuSO_5{cdot}5H_2O$), Iron(II)Chloride ($FeCl_2{cdot}4H_2O$). The best K/S value of dyeing temperature and time, all the fabrics were $100^{circ}C$, 90min. Silk fabric was dyed yellow(0.8Y 7.6/2.2) and nylon fabric was dyed reddish yellow(10.1 YR 7.4/3.0). Silk fabric and nylon fabric was changed greenish yellow on mordanting with $CuSO_5{cdot}5H_2O$ and $FeCl_2{cdot}4H_2O$ respectively. And the colorfastness of washing and dry-cleaning was improved by using mordanting agent(4~5 grade). Ultraviolet-cut ability(UV-B) was showed more 90% in dyed nylon fabrics.