A fuzzy system which can predict a dyeing rate constant and maximum adsorption in acid dye/nylon system was studied. In this system, pH control profile constant is an input variable and dyeing rate constant is an output variable and the simplified reasoning for fuzzy inference and gradient descent algorithm for optimizing the fuzzy system are used. The result showed that it is possible to predict dyeing rate constants and maximum adsorption with the online control of dyebath pH in acid dye/nylon dyeing system.