In order to clarify the dyeing mechanism of cellulose fiber with the reactive dye (Procion Yellow HA), the absorption and the reactivity of dyestuff were studied and following results were obtained:1) Diffusion coefficients calculated from Fick's equation depend on dyeing time at the initial stage. The uptake of the dye over the extended period is analyzed by applying the McGregor's treatment.2) Thermodynamic values on the absorption of the dye at pH 6.53 is similar to that of amino-azo benzen derivatives to cellulose. The rate of diffusion of the hydrolyzed Procion Yellow HA is constant at pH_??_9.57 and the absorption of the hydrolyzed dye decreases noticeably with increasing pH.3) The apparent reactivity of the dye with cellulose is very slow in comparison with that of hydrolysis of dye. On the finding that the diffusion of the dye is sufficiently fast compared with the reaction of the dye with cellulose.4) It was considered that the physical state of active center of cellulose fiber plays very important role in the reaction of the dye with cellulose.