The author proposed an equation of fading rate, expressed as ${Delta}E$=${Delta}E_{1{infty}}({{1-e^{-k}}_1}^t)+k_2t$, where ${Delta}E$=fading of dye (colour difference of N.B.S. unit), ${Delta}E_{1{infty}}$=extremity value of fading of molecularly dispersed dye, $k_1$, $k_2$=constants, t=exposure (hours). The validity of above equation, which is connected with the state of dye in fibre, was confirmed by experimental in a range of 2-3 colour fading in grey scale value for assessing change in colour visually, except the case of increasing the fading rate with the time resulted from the break-down of dye particles through the heat effect of light; it was also discussed about possiblity of evaluating of fastness to light of dye in early stage.