In general, the silk dyed with acid dyes is not good in fastness. To improve this defect, the silk dyed with Acid Mordant Blue 1 is treated with ferric nitrate solution. The results are as follows: 1) The amounts of iron absorbed on the dyed silk increase with the increase of temperature, concentration and pH of the ferric nitrate solution. 2) The development of the colour by the after chroming is accelerated according to the increase of treating temperature. In general, when the molar ratio between the absorbed dye and iron on the dyed silk is 1, the development of the colours reaches highest. 3) The colour fastness to light, washing, and hot water are improved by the after chroming, which is caused by the stable chelate formed with reaction of dye and ferric ion. 4) The amount of dye-Fe(III) complex is increased with the increase of Fe(III) concentration in the aqueous dye-Fe(NO$_3$)$_3$ solution which is the constant concentration of dye. With the reaction of Fe(III) on dye to the mole ratio 〔Fe(III)〕/〔dye〕=1, the absorbance of the maximum wavelength band 430nm of dye are decreased with the increase of the maximum wavelength band 590nm of dye-Fe(III) complex. As a results, it may be regarded that Fe(III) forms 1:1 complex type with the dye.