The fastness of dyeing with dispersed dyes depends on how deeply the particles of dyes are inserted. The microscopic test of the cross section of the dyed fiber has been widely used to know the distribution of dyes in fiber. Expecting that only the surface of polyester fiber dissolves in alkali treatment and leaves the inner part of the fiber unswollen, and a rigid stucture of Tetoron prevents. dyes from penetrating into inner part of fiber under ordinary condition, Alkali treatment of Cr treated Tetoron is continued till the combination of dyes with Cr at the surface of tetoron ends. under normal pressure.The polyester fiber used in this experiment was treated with K2 Cr2 O7 solution for 5hrs at 120°C and washed with water. Several portions of Cr treated tetoron, each 0.5g, were heated in 10% NaOH solution at 85°C changing the hours of the treatment and washed with water, and dyed with Sunchromine Pue Blure B. extra. Weight loss and absorption of dyes were determined. The relation between weignt losses and the time of alkali treatment is shown in Fig. I. The effect of absorption of dyes by Cr treated tetoron on the weight loss is shown in Fig. 2. Point A is a critical point where Cr is contained on fiber. From the weight loss at point A, the penetrated distance of Cr from the surface of fiber was calculated.