Natural gardenia and Japanese pagoda tree have been known as them of the most widely used yellow vegetable dyes. Cotton and silk fabrics(KS K 0905) were chosen as specimens. Those were dyed with natural gardenia and Japanese pagoda tree. The conditions of dyeing were varied according to the kinds of mordants, using mordants or not and the concentrations of the natural dyes. Sn, Al, Fe and Cu were chosen for the mordants. 5%(owf), 10%(owf), 15%(owf) and 20%(owf) were used as for the concentration of the natural dyes. The color differences were measured by the Spectrometer(UV-VIS-NIR) before and after dyeing. The results are as follows: 1. Deeper colors were proportionately shown up to the concentration of the natural dye. 2. Silk was shown the higher dye affinity than the cotton at the same concentration of the dye. 3. The color difference between the Sn-mordanted specimens and the Al-mordanted specimens were significant. When the concentration of the dye increased, the colors got darker, specially in b* value. 4. The color difference between the Fe-mordanted specimens and the Cu-mordanted specimens were significant in L* value and b* value. 5. The deeper or lighter yellow were controllable under the concentration of the natural dye and the mordants. 6. The most effective and the economic concentration of the natural dye was 10% owf through the experiments.