The extraction behavior of oligomer in the polyester fiber with hot water has been investigated. A B type of extraction system was used in an attempt to analyze running process of oligomer from the fiber into water, and the following expression was derived; Where, Ao : initial concentration of oligomer in fiber phase B : concentration of oligomer in hot water phase K : equilibrium costant k$_1$ : rate constant of running of oligomer k$_2$: rate constat of depositing of oligomer on the fiber surface from water t : treating time with hot water The results obtained are as follows; (1) When six kinds of domestic and foreign polyethylene terphthalate filament were treated with hot water at 110$^{circ}C$, 120$^{circ}C$, and 130$^{circ}C$, respectively, a linear relation between hot water treating time and log was confirmed, and the kinetic approach was supported. (2) Though thin layer chromatographic analysis of the extracts suggested that the oligomers were composed of some linears and cyclics, the authors have confirmed the presence of cyclic trimer mainly in the fiber surface deposits and water extracts, and the validity of extraction system was suggested. (3) As a heat of extraction with hot water, 25.9 kcal/mol, 36.3 kcal/mol, and 38.6 kcal/mol, were obtained for the three different fiber samples, respectively. (4) The relationship between rate constant of running of oligomer and critical dissolution time (CDT) was examined and polyester fiber crystallinity was found to have an effect on the rate of extraction with hot water.