A mathematical model for estimating the dynamic behavior of ethanol production from mixed sugars such as glucose and xylose is presented. This model was constructed by introducing the term for ethanol production into the diauxic growth model proposed previously by the authors. It was assumed that the first substrate, glucose, is metabolized by a constitutive enzyme and the second substrate, xylose, is utilized by an inducible enzyme. The synthesis of the inducible enzyme is controlled by the catabolite repression caused by glucose and with the induction caused by xylose as an inducer, and the xylose is then converted into cell mass and ethanol by the inducible enzyme. The parameters of the model were estimated from the experimental data in the medium containing glucose and xylose, singly or in combination in a batch culture. The application of the model was examined for experiments in both batch and continuous culture, with glucose and xylose as carbon sources. The calculated values, according to the model, corresponded satisfactorily with experimental data, such as cell growth, substrate consumption, and ethanol production, especially in the estimation of the lag times between the first log phase and the second log phase in cell growth and ethanol production.