Pichia stipitis CBS 5773 yeast cells were used to ferment the mixed substrates consisted of glucose and xylose to produce ethanol. The effects of aeration rate, initial substrate concentration and pH on substrate utilization and ethanol yield were evaluated. During batch fermentation, the oscillation phenomena in cell growth were observed at low aeration rate, whereas the diauxic growth at high aeration rate. The substrate utilization ratio and ethanol yield reached 95% and 0.46 g/g respectively under appropriate operation conditions. A modified unstructural model was proposed to simulate the diauxic cell growth, substrate consumption and product formation.