A mathematical model has been formulated to describe the heat transfer in liquid foods flowing in circular ducts, subjected to microwave irradiations. Three types of liquids with different rheological behavior are considered: skim milk (Newtonian), apple sauce and tomato sauce as non-New-tonian fluids. Each one can flow with different velocities but always in laminar way. The temperature profiles have been obtained solving the transient momentum and heat equations by numerical resolution using the Finite Element Method. The generation term due to the microwave heating has been evaluated according to Lambert’s law. Dielectric properties are considered to be temperature dependent.