The present work aims at the study of the buoyancy effects on entropy generation due to laminar mixed convection in vertical channels of different geometries. The geometries considered are namely: vertical channels between parallel plates, vertical circular tubes and vertical concentric annulus. Attention is devoted to investigating the optimization of the flow and heat transfer in such channels under the mixed convection condition via the minimization of the entropy generation. In this regard, the optimum values of the buoyancy parameter (Gr/Re)optimum at which the entropy generation assumes its minimum have been obtained and presented for both fully developed and developing laminar mixed convection flow. Moreover, the effect of other operating parameters of relevant importance is also discussed and presented