A dynamic model for a PZT actuated valveless micropump is presented in this work. The model couples PZT actuation with fluid flow in a flow chamber. Extended Bernoulli equation is used to describe flow dynamics in the inlet and outlet of the micropump. The dependence of the output pressure and flow rate on pump parameters is discussed. For low frequency actuation, the flow rate and back pressure increase as the PZT membrane thickness increases. The flow rate also increases for larger nozzle neck. The back pressure increases as the nozzle neck enlarges, reaches a maximum at about 80 microns, and then decreases when the nozzle neck keeps increasing. Such dependence becomes insignificant for nozzle neck larger than 200 microns. This model also predicts that the neck length has little influence on the back pressure and flow rate.