The dispersion behaviour of disperse dyes under the dyeing condition was assessed by turbidity ratio, which increased with decreasing particle size of dyes. The four different dye dispersions of C.I. Disperse Blue 56 and C.I. Disperse Yellow 54 were prepared by milling presscake for 1 and 25 hours. The turbidity ratio of the dispersions having large particle size rapidly increased above 10$0^{circ}C$, but that of metastable form decreased during cooling step. Whereas those of small particle size decreased as dyeing proceeded. It could be monitored that the particle size of dyes not adsorbed by polyester fiber were enlarged during dyeing process. The dyes with small particle size showed lower exhaustion value than larger ones at first, but eventually showed higher values above 13$0^{circ}C$.