A new method of analyzing an operation of an inkjet printhead by treating flow of ink in a nozzle and flow of ink in a chamber as separated models and connecting the boundary conditions of the two models was developed. This method was applied to simulate ink ejection by the multiple droplets, which had been difficult to analyze, and the simulation results successfully matched to the real experimental results.Since the gray scale printing by the multiple droplets requires ejection of small ink droplets at a high frequency of over 100kHz, the problem of this simulation was how to treat flow of the ink which vibrates at high frequency in a chamber. The ink in a nozzle was modeled as two-dimensional incompressive flow having a free surface and the ink in a chamber was modeled as three-dimensional compressive flow, then connected the boundary conditions of these two models. Both practical accuracy and practical computing speed were obtained by this method.This paper describes these models, compares the simulation with the experiments, and demonstrates the appropriateness of the simulation.