The appearance of gas bubbles in a valve-less micropump is generally undesirable as they can lead to performance deterioration, life reduction and even complete failure of the micropump. In order to predict influence of gas bubbles in valve-less micropump chamber on periodic driving pressure, the mathematical models, including volume change of chamber, continuity equation, effective bulk modulus and resistance coefficients of the nozzle and diffuser, are given to describe dynamic characteristics of piezoelectric valve-less nozzle/diffuser micropump. The influence of different gas bubble volume in chamber on the valve-less micropump periodic driving pressure is analyzed. Pressure pulsations with two gas bubbles into the valve-less micropump chamber are simulated and tested. Simulation and experimental results are given. Comparison of the results shows that the mathematical model and simulation method can handle the prediction of pressure pulsations accompanying gas bubble in the valve-less micropump.