Fluid flow characteristics and entropy generation in a circular diffuser/nozzle element are studied numerically. The flow is assumed to be isothermal, laminar and incompressible with constant thermo-physical properties. The velocity field, mass flow rate, and entropy generation are investigated for several half angles in combination with several values of pressure drops. The effect of diffuser half angle on the entropy generation is investigated and discussed. Furthermore, the effect of the half angle of diffuser on the diffuser efficiency and the effect on the rectification efficiency are explained. It is shown that there is an optimum operation half angle for which the diffuser efficiency has a maximum value. In the case of a micro-diffuser 1 mm long with an inlet diameter of 100 mu m, the optimum half angle was found to be 2.5A degrees. These results are based on several parametric simulations ranging from 0 to 7A degrees half angles and covering pressure drops between 500 and 2000 Pa.