In this study, effect of viscous dissipation on entropy generation for fully developed incompressible laminar flow mixed convection in vertical parallel-plate channels with asymmetric and symmetric uniform heat fluxes was parametrically analysed. Analytical equations were derived by using continuum approach and perturbation method. A specific MATLAB code was created to solve the governing equations. The effects of Gr/Re, the Brinkman number, and the ratio of wall heat flux on the entropy generation are presented and discussed. It is concluded that viscous dissipation has a significant effect on the entropy generation for mixed convection in heated vertical parallel-plate channels. Entropy generation resulting from fluid friction goes up, however, entropy generation due to heat transfer decreases by increasing of viscous dissipation. Consequently, it can be said that, this study will contribute to literature in optimising the systems that operates under mixed convection conditions in the vertical parallel-plate configuration.