A reduced-order model of a Microfluidic Transistor is presented. The transistor is essentially a long micro channel between substrate and a membrane that is pressure actuated. The proposed model captures steady (DC) and small signal (AC) behavior of the device in a manner analogous to standard semiconductor transistor models. The model is based on steady and perturbed unsteady solutions of the conservation of mass and momentum, coupled with an elastic model for the membrane. To improve the accuracy and to enhance the range of validity, the model is enhanced by numerical simulations of the coupled fluid-structure problem. The model predicts dependence of the transconductance on the pressure differentials across the membrane and along the channel. The proposed model also investigates the impact of flow inertia, among other effects, on the dynamic behavior of the transistor.