In this study, the second law analysis of thermodynamics is applied to two different microgeometries: microtube and microduct, between two parallel plates. Hydrodynamically and thermally fully developed flow with constant properties is examined. Microscale effects are included in the analysis in terms of the viscous dissipation, the velocity slip and temperature jump. Using the previously obtained velocity and temperature profiles, a parametric study is carried out to determine the combined effects of the Brinkman number, Br, and the Knudsen number, Kn, on the entropy generation. Entropy generation is shown to decrease with an increase in Kn while increasing Br results in increasing entropy generation.