The ejection performance and printing quality of a thermal inkjet printhead are significantly affected by bubble growth dynamics. This study investigated bubble growth dynamics and the ink ejection process under various heating conditions that are controlled by driving voltages. Numerical predictions are presented for bubble generating time, volume, temperature and pressure. The numerical computations suggest that a three-step voltage control method results in a larger ink-ejection velocity than a onestep voltage control method, even though the former generates less thermal energy. Through simulation, it can be shown that the three-step voltage control method actually has a larger heat flux rate from the printhead heater to the ink chamber when the ink is nucleated. Therefore a larger bubble pressure and a faster ink ejection are generated. This prediction is verified by the experimental results.