Mixotrophic culture of Euglena gracilis was carried out using a fed-batch culture system. Glucose and L-glutamic acid were added every 24 hours for 6 days to make the concentrations in the culture medium return to their initial values. We proposed a mixotrophic growth kinetics model to correlate a specific growth rate with a specific rate of energy consumption. In the model, the specific growth rate was proportional to the (1/n)th power of the specific energy consumption rate, and a corresponding yield parameter and a maintenance factor depended on light intensity. The specific growth rate in mixotrophic culture was correlated with incident light intensity by an equation that combined pseudo-photoautotrophic growth rate comparable to that of the modified Aiba model with heterotrophic growth rate. The optimum light intensity that produced the attainable maximum growth rate in mixotrophic culture could be estimated by the equation.